It does appear to be designed this way on purpose. I pulled this reasoning from the IMGBurn forums...
CD/DVD recorders (and modern CD/DVD-ROM drives) have a chunk of RAM that holds blocks read from the disc. Some drives provide a way to clear this out, some don't.
All drives need to have their block cache cleared out after writing completes and Before disc verification begins. If this weren't done, the files being verified could be read out of the block cache instead of from the disc itself, defeating the purpose of the verification pass. Also, some CD/DVD recorders need to have their recording buffers explicitly cleared between the "test" and "write" passes.
The most reliable, 100%-guaranteed-to-work approach is to eject the disc and re-insert it. Watching your CD/DVD tray open and close can be startling at first, but in general it's harmless.