Ownership and changes

[SirBanipal]

Dear all,
I have this problem: I just realized that I have some pages with the following specifications
-rwxr-xr-x 1 name lastname 1427 Apr 4 11:53 page1.xml
where I am not able to change nothing. And pages like this
-rwxr-xr-x 1 nobody nobody 2446 May 7 21:13 page2.xml
(note the nobody nobody) where I can change what I want.

Can you explain me what's going on? What am I supposed to do?
Thanks in advance!

My hypothesis is that these pages owned by "name" are pages that I uploaded few months ago when I tried to elminate a weird string of characters appearing on top of the page. Do you think it is possible to change the ownership to "nobody" again?