Can efgr improve drastically?

Hi, everyone my father was diagnosed with ckd and his efgr dropped from 44 to 29 over course of 6months, with strict diet and ongoing medical procedures and doctor visits we were able to restrict bp below 130 and sugar below 150 max (post diet), now i just got the test results done the efgr is back at 44 and serum is back to 1.66 from 2.34 with serum urea being in normal range we had restricted his diet to 4 vegetables no oil no protein nothing. What i am surprised is this sudden jump is this possible??