VA measure = value added, it means that the children have increased attainment levels hugely from what you say.
VAM is supposed to be as important as actual raw SAT numbers, in Ofsted gradings (I found a link that said that). Also, though I may be wrong, I think VAM is measured in a simple way as % gain from starting point. Thing is, children who come in with social advantages retain those & tend to improve at an above average rate. Meanwhile, kids with problems that limited their starting ability keep those same problems, so will have below average improvement by end of y6. So a high VAM for children with low intake scores, would be bucking the trend & suggests the school IS rather unusually good. At least for kids who start at a low base.
I think you are quite right to note that the feel of "satisfactory" vs. other gradings is probably very hard to discern for school parents. It comes down to invisible things like box-ticking, paperwork, written policies, luck on the day and SAT results.
Of course schools in a higher income area may well have a wealthier PTA, PTA often funds things like new facilities & equipment, so the newer/better looking school may come down to being in a wealthier area, which will tend to have higher KS2 SATs so it all comes together to improve gradings. At least, I suspect that Ofsted gradings are fairly biased by how spiffy, spic&span a school "looks", too.