Pitch fluctuations...

Tape stretches. Also tape suffers from what is known as wow and flutter. The older a tape gets, the worse the problem becomes. You don't get this with MP3's because it is physically impossible(so far) to unintentionally stretch a digital file.

Do you mean that the pitch of the song changes as it plays, or that it's just lower (or higher) than what you expect. If it's the latter, it's quite possible that they were simply tuned differently than you are.

Do you mean that the pitch of the song changes as it plays, or that it's just lower (or higher) than what you expect. If it's the latter, it's quite possible that they were simply tuned differently than you are. Or that the tape was sped up/slowed down to give the recording different feel

It doesn't change as it plays, its just consistently different. The first chord is a C, but playing the C chord to the mp3 file, it sounds different. Not in a "should be major/minor or 7th etc" way, even playing the bass note of C it sounds different. And not different enough to be in a different key altogether.

Its "don't look back in anger" by oasis, and I do feel slightly better that every tab I've found on the internet also tells me the first chord is in fact a C. It sounds fine when playing on your own, but with the cd its not.

The only thing I can suggest myself, is how my tuner can set A to a frequency other than 440Hz, so a sound with frequency 445Hz would be giving A on the tuner,for example. Why this would be done, I also don't know?

It's possible that the guitars might have been slightly out of tune when the song was recorded....have you tried playing along with the keyboard intro?

There are differences in what people consider A to be. Someone here. I think it was Greybeard, explained in detail the differences between the different A's. It is also possible that they tuned down to D#.

It has been a favourite trick of record companies to fiddle with the speed of their recordings. They modify the final cut so that the actual record comes out, say, 20-25 cents sharp of the actual key it was played in.

I put one tune through a spectrum analyser and discovered that every note (that I checked) was 23-25 cents sharp!

However, one of the things about tape is that the capstans that feed it past the heads rely on friction. Although made of rubber, they attract dust, get worn down and, very often, old tape has been played so often that it is very shiny on the back (the side that is "driven"), a recipe for slippage, which is what produces wow and flutter.

Many songs are recorded with all strings tuned down a half-step (Eb, etc.). Also the pitch that they tuned to may not have been "Concert Pitch" but some other pitch. Post recording, it's also possible that the producer thought that the song sounded better higher or lower, faster or slower in spots.

Or that the tape was sped up/slowed down to give the recording different feel
Yes, but I figured that with a digital recording they would just tempo shift without changing the pitch. Still, that doesn't mean they didn't pitch shift it, with or without tempo change. I guess it wasn't necessarily recorded digitally either.

There are several recordings where you encounter the pitch as odd. The studio recording of "Layla" is one. The pitch is off, but in live versions it isn't. The studio recording of "Under My Thumb" is another. It's much easier to match the pitch on the live version. I suspect that you're encountering a pitch change from recording speed. I think it's often done to affect the timbre of the vocals.