It's common for USB drives to reassign drive letters depending on the USB port you install them too. Windows assigns a hardware installation profile to each individual USB port.
What this means is when you install your flash drive on one USB port and then set the drive letter to R for removable it may assign a different drive letter if you plug that same USB flash drive into a different USB port.
There are also instances where if you install a similar or identical USB flash drive onto identical USB ports both will be reassigned the default drive letter as well. It really depends on the USB flash driver being used by Windows at the time.
It's unfortunately a side effect of "USB"… which stands for Universal Serial Bus. When something tries so hard to be universal it must run a complete reinstallation of device drivers when you plug the identical device into different USB ports. It typically does a good job remembering devices based upon their hardware serial number but will often reinstall them just in case.
Hope this helps clarify things. It's not just you, this is a very common issue with USB flash drives (and why I made my video I posted).
The problem is Windows XP didn't anticipate so many additional mapped network drives and external drives on computers so it doesn't do a very good job mapping drives after F:.
It's annoying, I completely agree.