What they can do in a nice quiet relaxed home, can be quite different from what they can do in a busy stimulating classroom.
There is working memory (eg holding numbers in your head for a calculation, or a sentence in your head to write down), and short term memory (holding steps 2 and 3 in your head whilst doing step 1), I understand they are subtly different.
Then there is processing speed - just being slower. If you interrupt their processing to prompt all it does is low things down as they have to start again.
Also audio processing - hearing and processing sounds.
Then of course dyslexia or dyspraxia which overlap in signs and can be co-morbid.
Both my DDs coped with home support at primary, issues started becoming more obvious in secondary and the gap was really obvious by the GCSE years. Ideally I should have pushed more for assessments in primary so it is great you are aware of issues now.
Getting ahead in class can be described as 'flip learning' (google it) and we found it helped too. They then went into lessons already knowing something, which made absorbing the details easier and gave them more confidence.