1ML shows larger number of nodes comparing to other lightning network explorers
Let's start by ignoring the 1ml case. It is perfectly reasonable that the number of nodes differs a little bit as the gossip protocol takes some time to forward the information. In particular there  could be private nodes having a channel to the node that is used  by the explorers and they could include the knowledge about such a node in their  counting...

So why is 1 ml counting almost twice as many nodes? As it is closed source we can only guess. I observed that 1ml also depicts closed channels for a node. So I could imagine that the number of nodes shown by 1ml also includes nodes which existed in the past by counting all nodeids they have ever seen in their database instead of counting the nodes from the gossip store. I did not find such an example quickly. Also the gossip protocol specifies to remove channels if they close on chain but has as far as I rember no rule for removing nodes. Maybe 1ml uses a different / secret lighing implementation. That never drops nodes from their gossip store.