A MIDI file can have more than one tempo
The initial BPM describes the beginning of the performance. A tempo map can speed up or slow down later; the table preserves each file event rather than reducing everything to a single average.
When a file does not specify a tempo at the beginning, MIDI playback starts at 120 BPM. We label that default explicitly. A missing time signature is also reported as missing rather than presented as a detected 4/4.