fiberglass roll insulation question

I have just rolled out a 6.25" thick roll of insulation between two 2X6 ceiling joists. Now the joists are actually 5.5" due to actual lumber measurements, but I was also shocked to see my 6.25" thick insulation still fall short of the actual lumber dimensions and not measure up to the top of the joists. I thought the 6.25" thick insulation would stick a little proud of the top of the joists. Any advice? Thanks.