What is going on is simple, and obvious to anyone with eyes - and I saw it happening in my kids primary school in a North Eastern city in England during that period of massive immigration through the 2000s.
The ability of the native population was on a bell curve, and the ability of immigrants was bimodal.
The Chinese (including Korean, Taiwanese, Hong Kong etc) were all in the top maths group and the top maths groups was mostly Chinese, one Jewish boy and a couple of locals. The bottom Maths group was I recall entirely Pakistani, Bangladeshi, Arabic, and African. Indeed the best students in most classes were usually Chinese - even though there were local kids of middle class professional parents in a higher proportion than usual.
When we got the children's attainment test scores, we were given the scores for the whole class (anonymously) to give context, and I was astonished to see they were actually bimodal (for 90 children - not symmetrical - since there were a lot of highly able children, but with two peaks and a point of rarity between). I asked the teacher why - given that this is a very unusual distribution for a school, but she had no idea, and it seemed that nobody had noticed or commented on the fact. The proportion of immigrant children was so high that it made this bimodal distribution.
The deep problem is that concepts such as 'immigration', and even more 'diversity', are actually calculated to conceal and confuse, by lumping together heterogeneous and indeed contrasting entities.
When we are forced to debate using these categories things seem much more complicated than they really are - thereby people cannot follow, and lose interest in the 'debate' - which is exactly the intention.