How to address strange author order of a collaborative paper within "equal distribution" section? I am PhD student in mathematics and am writing a joint paper with a computer science PhD student. As we will submit the paper to a computer science journal we discussed the author ordering and agreed to equal distributions for us PhD students and our respective supervisors.

The computer science student has now ordered the author names in an non-alphabetical order for us PhD students, which makes him the first author, and an alphabetical order for the supervisors, which makes his professor the last author.

Is this common practice in computer sciences? I can understand that it is their community, but this confuses me. I think it should be at least in the same order for both categories.

My supervisor told me to talk to the other PhD student. How can I address this issue without offending anyone?

How does equal distribution authorship in computer science work, anyway?

Edit: I just don't want to be fooled. I understand that being first author is important in computer science, this is why we decided to have both PhD students as co-first authors. Our supervisors are co-last authors. Having a non-alphabetical order for the PhD students and an alphabetical order for the supervisors seems to abolish this equal distributions again to me. I just have no idea how this equal distribution thing works and how to address this issue without messing up everything.
I am not sure if we PhD students will work together again, but I could imagine that our supervisors do.
Go with your supervisor's advice, talk to the Computer Science student and ask him/her, in a very gentle manner, why the authors were ordered the way they were rather than alphabetic in both sections.  This is the norm for equal distributions.  Can't imagine what reason the other student would have to go a difference way.  If he/she does not have a very good reason, ask them to reorder the names in alphabetical order.  If they do not comply, you can always ask them, again, in an email copied to the supervisors.  At all times, exercise grace and patience - being firm but gentle gets results much more often and burns bridges much less often.
PS: be very careful if you ask that student in an email.  Written communication on what could be a negative issue, will by default come across as confrontational.  Even if you are not trying to, they will usually see it as you confronting them on something they did that was non-kosher.  So if you do email him/her, make extra efforts to explicitly communicate a non-confrontational tone.  Maybe start by thanking them for their work in the paper and telling them how much you appreciated the opportunity to work on it as well.  Make your question about author order very light, do not point out his/her name came first, just that it was not in alphabetical order and you are wondering why that is, especially since you note the advisors are ordered alphabetically.