You can forget to bring your ride pass with you, or forget to purchase a ticket or accidentally purchase the wrong ticket. Or e.g. with Caltrain you can forget to swipe your clipper at the boarding station.

Jumping a turnstile isn't merely oversight; it's intentionally trying to avoid paying the fare.

Some people might be just trying to get away with not paying a parking meter deliberately, but we can't prove that when writing parking tickets. Most people probably just forget or cut it a little too close with their plans when they overstay a parking meter.

I thought you were comparing it to not having a valid ticket for transit in SF. I can see that it would be harder to prove intent in the parking meter situation.