It comes down to the formatting of the data on the disc.
Much like a hard drive there are overheads for things like error correction and the file tables. Using the audio frames format calculation (with 75 frames of data per second)
74 minutes CD = 74 min. * 60 sec. * 75 = 333000 blocks
80 minutes CD = 80 min. * 60 sec. * 75 = 360000 blocks.
With the regular "Data Mode 1":
74 min CD = 333000 blocks * 2048 bytes = 681984000 bytes = 650.4 MB
80 min CD = 360000 blocks * 2048 bytes = 737280000 bytes = 703.1 MB
Now with the "Data Mode 2" have:
74 min CD = 333000 blocks * 2336 bytes = 777888000 bytes = 741.9 MB
80 min CD = 360000 blocks * 2336 bytes = 840960000 bytes = 802.0 MB
With "CD-ROM-XA Mode 2 Form 2":
74 min CD = 333000 blocks * 2324 bytes = 773892000 bytes = 738.0 MB
80 min CD = 360000 blocks * 2324 bytes = 836640000 bytes = 797.9 MB
The error correction is important for computer data, less so for the video stream data that would normally use mode 2 or XA.
So you could write more to the disc by changing the format you write, but you would lose reliability of the data. The discs could store 800 Mb - just it's not advisable if you actually want to access the data again.