zx2r starting issues....kind of!!

my brothers are learning on a 1990 zxr250 (a) or zx2r, or whatever the fark it is. we're having trouble getting it started.
sometimes it'll keep winding till we kill the battery. rarely it'll just fire up immediately. If we try push starting it instead it WILL run immediately, and easily aswell.
Why would a bike be so difficult to electric start (and the starter is winding the motor at a decent pace), but so easily bump started?
When i say easy to bump start i mean easy to bump start. Always runs first time.
Any ideas?
sometimes it'll keep winding till we kill the battery. rarely it'll just fire up immediately. If we try push starting it instead it WILL run immediately, and easily aswell.
Why would a bike be so difficult to electric start (and the starter is winding the motor at a decent pace), but so easily bump started?

Any ideas?