Because Hexdecimal is another math-system.
What he said. For the record, here's a small recap of the numbers in hex system:
In decimal, you'd get such a suite of numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16.
In hexadecimal, that would give this: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10.
The reason why you can't get more than 255 minutes is because the highest possible entry for an individual byte is FF, which equals 255 in decimal form.