There isn’t a bug in Nero regarding audio CD file sizes. 
CDROM data throughput at 1x is 75 sectors per second. To find the capacity for any time rating:
(75 sectors x capacity minutes x 60 seconds x (sector bytes / 1,024) /1,024) = MB Capacity
The minutes and data capacity on CDR labels is only for ISO 9660 data recording which uses a sector size of 2,048 bytes (2 KBytes) for Mode 1 and Mode 2 Form 1 . The balance of the total bytes available (2,352) is devoted to error correction.
For a 74 minute data disc:
75 x 74 x 60 x 2 KB = 666,000 KB /1,024 = 650 MB (rounded)
An 80 minute (specified standard is 79:59.74 min.) data disc = 703.1 MB (rounded)
(S)VCD’s (ISO 9660 Mode 2 Form 2) use a sector size of 2,324 bytes. The difference in the sector sizes for ISO recording is due to space allocated to error correction capability. Since their contents are not critical data, (S)VCD’s basically have no error correction. A 74 minute (S)VCD can hold about 738 MB. An 80 minute (S)VCD can hold almost 798 MB.
For a 74 minute Mode 2 Form 2 data disc:
75 x 74 x 60 x (2,324/1,024) =
75 x 74 x 60 x 2.2695312 KB = 755,753.9 KB / 1,024 = 738 MB (rounded)
An 80 minute (S)VCD disc = 797.8 MB (rounded)
Audio CD®’s use the entire sector size available of 2,352 bytes without the extra error correction of the data mode. Because of this added information in each sector, a 74 minute Audio disk can hold about 747 MB. An 80 minute Audio disk can hold about 807 MB.
For a 74 minute audio disc:
75 x 74 x 60 x (2,352/1,024) =
75 x 74 x 60 x 2.296875 KB = 764,859.4 KB / 1,024 = 747 MB (rounded)
An 80 minute audio disc = 807.4 MB (rounded)
Information on error correction and other sector allocations can be found at The Sector Formats of Compact Discs.
For your 78 minute audio disc:
75 x 78 x 60 x (2,352/1,024) =
75 x 78 x 60 x 2.296875 KB = 806,203 KB / 1,024 = 787 MB (rounded)
As you can see, for a full 78 minutes, an audio disc should contain about 787 MB. 
MB’s count, you just have to count the right type of recording. 