hi
you're right it't not that accurate, it is a bit subjective, the bone that they measure from at the bottom is 2 cms in width generally anyway so sometimes they can go for a different bit on the bone! Also baby's position can alter where the top of the uterus is. Up until a few years ago nobody measured, midwives went on instinct and experience - but it was decided everyone should have a baseline to go by - its also important for midwives to use instinct and experience alongside the measuring! When you see lots of pregnant tummies you get an eye for whats looking large or tiny or ok.
also, baby's can have abit of a spurt and then slow down and even out. I think its likely that since your last appointment the baby has grown at a faster rate despite all of the above - although it may well be ok and will even out - its still important you have the scan, but i wouldnt loose sleep over it as its so common at this stage to have scans for being big/small and more often than not all is ok x
