Hamish,
Yes, certainly in terms of progress, it is very possible that a child may make different amounts of progress in successive years.
Whether that, arithmetically, will translate into final levels that are above / below / in line with national expectations at the will depend.
For example, a child who finishes Year 2 on a 2B is reaching national expectations.
In the two years to Year 4 (I'll do this in 2 year steps as otherwise we encounter the '3 sublevels in 2 years' problem) they might make less than expected progress and perhaps make only 2 sublevels rather than the expected 3 and thus end on a 3C.
In the two years to Year 6, the child may make 4 sublevels of progress (ie make above expected progress) but end up on a 4B which is reaching (not exceeding) national expectations.
Or for example a child who finishes year 2 on a 1B - Even if they exceed expectations for PROGRESS up to Year 4 and up to Year 6, and make 2 sublevels of progress every single year, they would still end up below national expectations at the end.
Or a child who finishes Year 2 on 3B (which they should to get a Level 3 in that year) - even if they make only a single sublebel of progress over the next 2 years, they will still be above national expectations at the end of Year 4!
Which is why I say it is important to look at progress, not absolute numbers. Not to expect the same progress every year, no, but to check that all children are making the maximum progress they are capable of e.g. a high ability child not being left to 'drift' or a lower ability one to flounder.