Why is av.time_base an int but container.streams.video[0].time_base is a Fraction?

[thomas-swe]

If I evaluate av.open(url).streams.video[0].time_base, I get the Fraction 1/30000 (for one particular video), which I understand to correspond to a unit of time (one thirty-thousandth of a second).

On the other hand, av.time_base is a large int, 1000000, which presumably corresponds to microseconds.

Intuitively, I would have expected these to be expressed in the same way, i.e. I would expect av.time_base to be a Fraction equal to 1/1000000. Indeed if using seek without specifying a stream then it appears that the offset is meant to be given in terms of multiples of 1/1000000. I read through the docs, but was unable to find an explanation for this. Could someone please help me to understand the reason for this discrepancy?