I seem to have a problem with pre-boil gravity readings with a hydrometer. I think it's hard to get a well-mixed sample, and my readings often tend to come out lower than anticipated.
On one batch, I reacted to the lower-than-expected mash gravity reading, added some DME and ended up with a higher FG and higher estimated ABV than I had planned on.
On the next batch I also got a lower mash gravity reading, but ignored it by making no additions to boost gravity and my FG reading came out spot-on to my original calculations in excel.
As long as my FG readings match by original calculations, I plan to ignore mash gravity.