Zoho Sheet inserts strange values while dragging down a formula

Zoho Sheet inserts strange values while dragging down a formula

Dear Zoho Team,

I've just made some experiments and can't figure out what's happening.

I have a cell with a value '2011-01'.  I drag this cell down by mouse and expect it would be something like 2011-02 (or at least 2011-02, as in OpenOffice by default).

But it becomes '202    - yes, with that apostrophe at the beginning.

Furthermore, if i have a value 2011-100 and the drag it down, everything will be correct, next cell will have 2011-101. 

First of all i thought it was related to date somehow. But having value like 2011-999 (that can't be date, right?) also makes it correct. 

I've been trying to change cell format but no success.

So what is wrong with that 2011-01?

Thank you.