Well POBI did identify quite a large component that looks possibly Slavic in East Germany, although it has a significant presence in many other areas but not in the Netherlands, Denmark or north,north-west Germany. I remember in a study about the Flemish, R1a was quite common in inland areas but dropped very low (1% in one area i think) along the coast, while R1b-U106/L48 and I1 were much higher along the coastline. So it may be that you see it in one place and not in another as it perhaps follows to some extent the old division between more inland Germanic areas and the North Sea Coastline, seeing as Rostock is in the western Baltic and directly south of Sweden a high R1a result is pretty expected, and in areas of central Germany is it is also perhaps not so surprising given that this study has shown that in Flemish areas R1a drops off somewhat significantly compared to the southern parts of the North Sea where R1b increases.
I think there was a post about it on this forum somewhere.
http://www.fsigenetics.com/article/S1872-4973(13)00101-4/abstract
This kind of fits in with the lower frequencies of R1a from the Netherlands to Denmark (6-12%), given that they are coastal north sea locations - even though it is just about Flanders, lower frequencies of R1a are seen in areas along the coast from Belgium to Denmark (although not in the Baltic). Just kinda makes sense to me that different areas within a cultural group have different genetic makeup, at least in Y-dna. Given that it's unlikely that there is a drastic change from 20-30% R1a to less than 10% along the Dutch border, it would seem more likely that it gradually decreases as one goes towards the coast, and R1b-U106 increases, which fits in with Flanders.