Why 51% is explicitly mentioned to get control over the network?. Why not 50 or lesser ?. Anyway there is no central authority to decide, whether 51% is attained or not?.
With over 50% of the hash power you have the majority of the nodes under you control.
It's not about anyone deciding whether 51% has been reached or not. It's about the fact that if you control more hashing power than the rest of the network you can start mining on your own side chain and eventually it will be the heaviest chain and all miners will switch to mine your chain.
It's a bit like rolling a dice: if persons A and B are rolling a dice and A wins if he gets 1, 2, 3 or 4 and B wins if he gets 5 or 6 it's obvious who will win more games in the long run.
You should read more about how the network reaches consensus and how mining works to better understand the issue.