You might be on to something. 3 years of NBA experience. Still only 21 years old. And he is taller than you think.
3 years ago at the NBA draft combine, so he was 18 years old and maybe not finished growing - He was 6-8.31 in bare feet, 6-9.5 in shoes, 7-6 wingspan, 9-3.4 standing reach.
His combine measurables are almost identical to Elton Brands, one inch longer wingspan/standing reach.
Amare Stoudamire was 6-8.5 at the combine, 7-2 wingspan, 9-1 standing reach. Brian Grant, Juwan Howard, Adonal Foyle all measured 6-8.5 in bare feet.
Al Horford is 6-8.75 but only has a 7-1 wingspan and a 8-11 standing reach.
I dont think Biyombo is too short for center and his long arms more than make up for any lack of height.