Molecule similarity measurement

I'm not a chemistry specialist, I'm more from informatics, and I have a question concerning molecule similarity measurement.


Let's taking these two molecules:
mol1: c1(cccnc1)C(O)=O
mol2: O=C(O)c1ccncc1


If I compute the Tanimoto similarity of these two molecules, I find 0.42, now, if I use ChemMine tool, it shows:
AP Tanimoto: 0.636364
MCS Tanimoto: 0.6364


My first question is: what's the difference between Tanimoto, AP Tanimoto and MCS Tanimoto ? Which one is the 'true' similarity measure ?


Secondly, let's take a third molecule, mol3: NC(=O)c1cccnc1, if I compute similarities between mol1 and mol3, I have:
Tanimoto: 0.633
AP Tanimoto: 0.636364
MCS Tanimoto: 0.8000


So I find that mol3 is more similar to mol1 than mol2, is it correct ?


But if we look these three molecules representations (the picture shows mol2, mol1, mol3):
[![enter image description here](https://i.stack.imgur.com/uXvqy.png)](https://i.stack.imgur.com/uXvqy.png)


We can see that mol1 and mol2 differs just by the position of one N atom, and mol1 and mol3 have one difference atom (OH != H2N), so my last question is: is it 'chemically' true to say that one atom difference is more similar than on atom at a difference place ?


The reason of this last question is because, intuitively, my non-chemical mind find the mol2 more similar than mol3 to mol1, as there is just one atom on the other side of a bond, whereas in mol3 there is an atom which is different.


