I like the hypothesis of the air absorbing under pressure, then releasing as smaller, more broadly distributed bubbles that can escape.
You get a spongy pedal because a small section of line has a concentrated amount of air fillingf the entire diameter, making it airbound. Push the air into solution and it's now distributed through the whole volume of fluid. If the air then comes back out as smaller bubbles that are much smaller in diameter than the line, they can find their way up to the reservior & leave through the vent.
No data to support it, but it appears to make some sense.