Mob spawner have a collision that is a bit smaller than it should be. This bug also exists for trapdoors. MCPE-13451 For trapdoors it has been fixed in a beta, but mob spawners haven't been mentioned so mob spawners might still be affected in the beta.

How to reproduce:

1. Place a full block like for example stone.
2. Place a mob spawner above it. 
3. Try to jump on top of the stone. 

Expected result: You can't stand on top of the stone.

Actual result: You can stand on top of them.

Like it does with trapdoors it also works horizontally and not just vertically. You can for example place a mob spawner next to a full block like stone and when you look at the coordinates at the top left and you'll see that you are lower when standing on top of the spawner. And if you are below the mob spawner and fly against it you can walk against the block next to it.