This is not true. E.G. Lets take a level 12 Human Pit fighter, 18+3 str, 15+3 wis, 14+1 con. plate, plate spec, armor spec.
He will have, at level 12 assuming a +3(Githzeri) Plate an AC of 10[base]+6[lvl]+8[plate]+2[gith]+3[enh]+1[spec]+1[pit fighter]+2[shield]= 33 AC.
Now lets take a level 12...