The MMA's Standard MIDI-File Format Spec says:
The third word, <division>, specifies the meaning of the delta-times. It has two formats, one for metrical time, and one for time-code-based time:
+---+-----------------------------------------+ | 0 | ticks per quarter-note | ==============================================| | 1 | negative SMPTE format | ticks per frame | +---+-----------------------+-----------------+ |15 |14 8 |7 0 |
[...] If bit 15 of <division> is a one, delta times in a file correspond to subdivisions of a second, in a way consistent with SMPTE and MIDI Time Code. Bits 14 thru 8 contain one of the four values -24, -25, -29, or -30, corresponding to the four standard SMPTE and MIDI Time Code formats (-29 corresponds to 30 drop frome), and represents the number of frames per second. These negative numbers are stored in two's complement form. The second byte (stored positive) is the resolution within a frame [...]
Two's complement representation allows to sign-extend negative values without changing their value by adding a MSB bit of value 1. So it does not matter whether you take 7 or 8 bits.
In practice, this value is designed to be interpreted as a signed 8-bit value, because otherwise it would have been stored as a positive value.