Could be the doc. While you'd think their equipment was simple enough, my dr office measured me about 2 years ago and came up with 5' 9", having been 5' 11¾" since 20 I was dubious and so I checked myself and came up with 5'11"+.
Just a couple months back my new doc did another measure and called me 5'10", I called bs, asked her to chexk, she did and still had 5'10". So I went home, stood by wall, had my gf put a carpenter square to wall & my head and make a mark and sure enough 5' 11½".
I don't mind some spinal compression costing me some height, but since they're basing recommendations on BMI, next time I go to the docs I'm bringing my own tape measure to check their equipment.