Dlaczego "gets off", a nie "gets out" w zdaniu "The child often falls when it gets off the bike"?

GET OFF: zejść z czegoś, zdjąć coś (na dół)
1. To move from being on top of (something) to not being on top of it.
Get off your chair and help me.
Get off your rear and help me.
2. To move (something) from being on top of (something else) to not being on top of it.
Could you get the book off the top shelf for me?

GET OUT: opuścić coś (wnętrze czegoś), uciec
1. To leave or escape
In case of fire, get out by the nearest exit.
2. To help someone leave
We must get the children out first.
To leave a vehicle such as a car. (Note: for public transport, get off is more common.)
I'll get out at the end of the road and walk from there.

Zerknij tutaj na to interesujące zestawienie:
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv253.shtml