My only explanation is that Slovenia has had much more settlers in the region. (as well as possible indigenous peoples) and they may be mostly Slavic in genetics; but some or more or less of them have Germanic, Italic and possibly archaic Celtic admix.
What strikes me as very interesting about the Yugoslav people (south slavs) is that they tend to be, both male and female; extremely tall or above average in height for most Europeans. (kind of like Scandinavians or Dutch/Germans) however they lack similar features to Nords/Germanics - Perhaps it is the abundance of the I lineages, in the region.
Sorry if this explanation was too complex. (but to my knowledge; Slovenia and Hungary is a major European crossroads area - just like France and Italy. And lesser Poland, Scandinavia and Ireland -- where more homogeneity is seen.)
Edit: Also, from my observations ... This may also explain why Finns carry N1c but are also very tall. It may be haplogroup I. People with haplogroup I (both I1 and I2) often show very tall height. While R1b (from my observation) tends to have the shortest descendants. (Portugal for example: the average man is height 1.72m) R1a seems to be in the middle.
Also, European Mythology (or folk-wisdom) has always characterized the Celts as a "small" people. (In contrast to the Germanics and especially Nords,who are seen as taller.)
The most I read this thread, the most I doubt of my eyes.
Where did you red (or guess) Celts were "small" in European Mythology (is there somewhere something like an "European Mythology"? Are you speaking of 'korrigans' of the Breton legends, or of the 'leaprechan's' (sorry for the spelling) of Ireland?
here are only some sources, Julius Caesr among them, saying the Celts were rather taller than the rather short Romans, but varying by regions and as a whole a bit shorter than the Germans. Nothing scientific here but surely some reality, nevertheless.
...
Y-I seems to me linked to Late Paleolithic/Early Mesolithic people, over the whole Europe. I1 were surely more northern but was not "scandinavian" because nobody lived in far North during LGL. And some found an Y-I1 among farmers in Neolithic Hungary.
I think Y-I2 was more spred (as well the ancestors of I2a1a, I2a1b and I2a2), and already separated by distance in more than a group, what can be verified by its today subclades broad and specific distribution.
Since we are doing bets, I suppose the most of I2a1b was centered around Balkans and Carpathians, in HGs world. A first flow of Y-G2 farmers from W-Anatolia had pushed a lot of them. Concerning Cucuteni-Tripolye final period, I wonder if a second Neolitic wave mostly Y-J2 and aware of some metal realities did not take the strong side upon a mix of first Y-G2 farmers and of pre-Neolithic Y-E-V13 - from Adriatic shores but expanded into the Central Balkans rivers network and became farmers too. The Y-I2 were denser in the highlands, refuges and in the Dniestr region; osmosis came after in Tripolye: the mixed males had a strong 'mediterranean' component when the females were more of 'archaic' types if I red well. It is even said (in Wiki?) that the 'archaii' element was rather stronger in the previous phases than later. So, reinforcement from southern people? Before colonization of the Steppes?
From this scheme we can suppose the Northern part of Tripolye was inhabited by more Y-I2 people of more 'archaic' types. It's this northern part which stayed aside the first (I-E?) moves westwards from Steppes (boomerang effect? Y-R1b dominant new people) and received later the most northern Y-R1a Balto-Slavic tribes before becoming the Slavs? They stayed aside because they were between mountainous North Carpathians and marsh lands, not too interesting at first sight. Among them the first rulers Y-J2/Y-EV13/Y-G2 lost weight in %s.
All the way, the lost in 'gedrosia' among Balts and Slavs cannot be pure hazard. It seems the 'gedrosia' rich tribes from East took a more southern road and colonized Europe before more northern Y-R1a imposed themselves in continental East-Europe, what they had begun to do in Northern Plain.
Concerning Y-I2a1 in ex-Yugoslavia and possible autochtony, I wait more clues about subclades. It could be possible there were very more Y-E1b-V13 people there...All bets.