With Mac OS X 10.6 Snow Leopard (2009), Apple changed the way file sizes (disk sizes, folder sizes) are represented when using ‘kilo’, ‘mega’, ‘giga’, etc. (Base 2 vs. Base 10.) Unfortunately, the meaning of giga is now different between macOS and Toast. Regardless of what anyone thinks is better, it is confusing if it differs from application to application.
A DVD-R with capacity of 4,700,000,000 bytes can be filled with folder of 4.7 GB (minus overhead) as calculated by macOS, which is the same as 4.377 GB as calculated by Toast (4,700,000,000÷1024÷1024÷1024). Toast has some threshold byte numbers build-in, but I don’t know their exact value.
This is the same for all versions of Toast (up to and including v16).
In general, the reliability of reading back from disc, is a bit worse on the outside, so it pays off to not fill a disc to maximum capacity. So don’t feel bad about your 94% discs.