I think we're talking at cross purposes and perhaps best to lay out scores observed:
Hi Well then let's explain the system using your data:
2c, 2b, 2a - 2b, 2a, 3c - 2a, 3c, 3b - 3c, 3b, 3a
partly because I think we're speaking at cross purposes: I would take your 2a sub-level (end first year) as achieved.
so rolling back system:
Year 3: 2c - 2a
Year 4: 2a - 3c/3b (working to 2a again from start of year (although already achieved by pupil end of previous year. end of year target usually presented as 2/3 sub-levels ahead so in this instance 2a to 3c/ 3b for end of year target - open ended & really up to child - if they report 3b then 3 sub-levels achieved but rolled back next year)
Year 5: 3b - 3a/4c (working to 3b again from start of year - this could be real or rolled back (i.e. child could have achieved 3c or 3b) - again open to child to achieve 2 or 3 sub-levels)
Year 6: 3a to 4c/4b (may involve roll back (so working to 3a at start of year) - may also be presented as secure 4c working to 4a at end of year. Rolling back from 4b (which was what I suggested to Labro can be about doubts about achieving boundary from NC Level 4 to NC Level 5c).
It's not unfair to the teacher or pupil. It's possible to achieve 3 sub-levels (actual 2 sub-levels) in a given year and to achieve full 2 NC sub-levels.
Not applied to 100% of all subjects (maths/ science/ English) & not applied to 100% of pupils.
It's a way of moderating workload across a year - whereby children where there is doubt about achieving 2 full sub-levels start back at the beginning for last sub-level achieved.
Hope that makes more sense
Again teachers writing or indeed just reading this - if there is a website from the government which specifically states that English primaries are legally required to achieve 3 sub-levels progress per pupil per year for English/ science/ maths please, please, please direct me to it. Our school firmly maintains they are only required to achieve 2 NC sub-levels (APP level presumably match what Kent has here: www.kenttrustweb.org.uk/UserFiles/ASK8/File/School_Improvement/Ofsted_Ready_File/1_-_Achievement/KS1_2/What_is_Standard_Progress.pdf, but our school does 'endeavor' to achieve 3 sub-levels progress in Reading, Writing & maths for pupils. No statement on science & no scores reported to us Y3 - Y5 (possibly will be on KS2 SATs - don't know yet DD1 in Y5).