@FTEngineerM at the time DS was born (2018), nobody actually flagged that he was medically considered a 'low birth weight' although he was obviously small.
I did have growth scans due to low fundal height (although the ultrasound always indicated a higher weight than he actually was) and no other issues (e.g. placenta, cord function) were picked up.
Based on on the above, I queried why I was being referred to a consultant due to low birth weight that I had no previous knowledge of! My midwife said that the birth weight wouldn't have been mentioned/flagged due to being considered 'normal range' at the time. Apparently the guidelines have changed and the weight is considered 'low' now. This might be the cause of the discrepancy for you?
Just for info, DS was 6 lbs 15 oz (3.14 kg) when I gave birth at 41 weeks +4 days. I double checked the graph in his red book and he was actually plotted as just under 9th percentile at birth, then followed the curve pretty accurately until he was 3 months old (kept leaping up until he hit 75th at a year old)!
I'm not sure whether the percentile charts would be plotted differently now due to the new guidance (from what you've said, it seems possible)? A question for someone medically qualified, I guess!
It might be worth asking your midwife what she thinks (I'm not sure why I was referred to a consultant but you weren't...) however I'd be inclined not to stress too much if your first pregnancy/baby was otherwise healthy and no underlying issues were picked up.
I had a telephone appointment with a registrar who explained that the additional monitoring/aspirin this pregnancy is precautionary just in case there were underlying issues causing the low weight last time (seems unlikely as nothing was picked up on scans) but that it's entirely possible (indeed likely) that I just have small but healthy babies.I have to admit that I do feel this pregnancy is being over medicalised (based on healthy DS and pure non-scientific gut instinct!) but on balance there's no harm in taking the aspirin/having the extra scans, so I'm going with the flow...