Ok, the samples of BA Balkans which are more like Cucuteni are from Early BA from Bulgaria. The samples with more steppe are from Late Neolithic from Croatia.
This makes complete sense. Old Neolithic/Chalcolithic cultures like Cucuteni-Trypillian and Varna in the eastern Carpathians and Balkans were densely populated and acted as bulwark against Steppe invasions. Therefore Steppe admixture levels in those regions are lower. EHG, WHG and Steppe admixtures in those regions probably progressed slowly over time by intermarriages with neighbouring HG and Steppe pastoralist populations.
The samples from Croatia could be seen as part of the
large-scale migration from the Pontic Steppe to the Hungarian Plain, which was an ideal environment for cattle pastoralists and horse riders. I have explained in my R1b history, in agreement with what David Anthony wrote, that the
early Steppe incursions into the Balkans were raiding and pillaging expeditions that caused the progressive decline of Old Europe, but didn't leave any major Steppe settlements. Steppe invaders only took over politically and set themselves up as the new rulers in local communities, so that Steppe lineages would principally show up in elite burials like in Varna.
The real Steppe migrations with their families, carts and herds followed the Danube until Hungary and northeast Croatia, then moved into Germany, Bohemia and western Poland to found the Unetice culture. Some of those who arrived early eventually continued west beyond Germany, reaching France and Britain by 2300 BCE, when Unetice started around Germany.
Here is one relevant passage from my
R1b page.
"The expansion of R1b people into Old Europe was slower, but proved inevitable. In 2800 BCE, by the time the Corded Ware had already reached Scandinavia, the Bronze Age R1b cultures had barely moved into the Pannonian Steppe. They established major settlements in the Great Hungarian Plain, the most similar habitat to their ancestral Pontic Steppes. Around 2500 BCE, the western branch of Indo-European R1b were poised for their next major expansion into modern Germany and Western Europe."
However the three Bronze Age Croatian individuals tested date from 1700 to 900 BCE, which is long after the Yamna-descended tribes passed through the Hungarian plain. Nevertheless some seem to have made their way south until southern Croatia (Veliki Vanik) and may have been the long lost tribe of the
Illyrians. This is the first evidence we have of Indo-European lineages in Illyria prior to the Slavic migrations that replaced most of the male lineages by I2a-Din and R1a lineages. Unfortunately, only one of the three BA Croatian samples was male, so there is only one Y-DNA sample and it belongs to
J2b2a-L283. It's possible that a later Steppe migration that the one that founded Unetice brought J2b2a to the Dinaric Alps. Anyway these three individuals were undeniably Steppe-admixed and the J2b guy possessed a typical Steppe mtDNA (I1a1) also found in Unetice.
Note that the sample with the highest Steppe admixture in the Balkans (I2163) is from Middle-Late Bronze Age Bulgaria (c. 1700 BCE) and belongs to R1a-S224. It is the only MLBA individual tested and he is contemporary to the Srubna culture, where he surely originated. This confirms that that Srubna was a predominantly R1a culture, as opposed to the earlier R1b-dominant Yamna culture and probably also Catacomb culture.
NB: It's just a detail, but the samples tested are from the Trypillian culture, not Cucuteni. Although they formed the same culture, Cucuteni normally refers to the part in NE Romania and Moldova, while Tripolye/Trypillia is the Ukrainian part.