It's not a big deal and pretty much par for course with Marlins. Usually it's the front barrel band that pulls the mag tube up towards the barrel. Try removing the front sight (be careful with the screws!), and slide off the front barrel band. The mag tube should drop down and have lots of free play, even with the foreend barrel band in place.
Now bring the mag tube up towards the barrel so that it is located where it would be if the front barrel band were attached. If you feel the mag tube contacting the barrel and it requires increased force to move it into place, then this is the "bind" that is often spoken of as the cause of stringing in tube fed rifles as the barrel heats up.
The solution is simple and just involves relieving the inside diameter of the front barrel band so that no additional pressure is needed to locate the magazine tube under the barrel.
Pretty much every Marlin I've seen, except for some that have extended magazine plug tubes that bare against the barrel have slanted gaps. It's really just cosmetic, so shoot the rifle for a while and see how it groups. If you get stringing, then maybe look to adjusting the magazine tube location.
There's lots of great info on the web about tuning Marlins for accuracy... the one that is oft quoted is from maurader (do a google search).
Cheers,
Stirling