Warped/Misshapened Cylinder test print

Hello, I'm working with a FlashForge Creator Pro and FlashPrint (I'll be moving to Simplify3d soon), and I've been trying to work with getting my end result print dimensions as close as possible to the design dimensions without having to change the actual design dimensions to account for various printing nuances (like horizontal expansion). However, after doing a test print of 9 differnet sized vertical cylinders (each cylinder increasing in ID by 0.1mm from the last), I have noticed that the bottom of each of these 15mm tall cylinders has an inward-shaped warp/indent near the bottom. The top of the cylinders measure within reason (wall thickness, ID, etc OK). It's this inward "bulge" that has got me wondering where to poke around next. Any suggestions as to what I might be looking at here?Thanks!