You will probably see a more rapid increase as your overall fitness will be increasing as well.
Its one of those things that you get out what you put in - the more pull ups you do the bigger the results you will see.
Personally I hate pull ups as (for me at least) it does seem to take forever to see any improvement. Once you can do a couple of extra pull ups try and do your max chin ups and you will be shocked how much your upper body strength has actually increased.
In a pull up the load is against you pulling your arms into your body therefore the muscles that pull your arms in are being used, these are not really used in a press up because gravity does all the work, its extending your arms which has the load resistance which is not there in a pull up.
My pull-ups aren't my strongest point. I only just cracked out 5 on PRMC. I never really have an opportunity to train them. I have literally nowhere in the house to do them so I have to to go the gym for the pull-up bar.
Like everyone on here has said it's different muscle groups,
I can only do about 15/20 marine press ups,
and I can do 4 pull ups,
and what's also a bit weird is I can do 17 chin ups,
wish I could do them at PRMC *text deleted*.