They never get it right.
DS’s red book got his birth weight wrong. At three weeks our health visitor plotted his weight at the four week mark. At 6 weeks the GP plotted his weight, length and head circumference at the 8 week mark. At 12 weeks another HV plotted his weight at the 15 week mark. At 28 weeks a third HV plotted his weight at the 26 week mark. At 44 weeks (10 months) the same HV recorded his age as 43 weeks and plotted his weight at the 43 week mark and his length and head circumference at the 11 month mark.
I should add that when he was weighed at 12 weeks it looked like he’d lost weight due to dodgy scales. Always take a 1kg bag of sugar with you when you take your baby to be weighed because sometimes those scales are way out.