I would say look in the PIC, but you probably already did.  It should have all of that, otherwise, I'm sure someone around here knowns.  You TAS shouldn't really be that different based on temperature and altitude unless your climbing a couple of thousand feet.  Are you sure you had the most current winds aloft?  I've done cross country flights where the winds have changed dramatically in the couple of hours between when I left and when I landed, that might have happened, otherwise, I don't know. ???

Well, a minute is pretty close for something like that in my book.  My instructor only ever calculated times to the nearest minute because there was really no chance of getting to each point exactly on time.  Even for my check-ride the examiner was impressed that I got to my check-points within two or three minutes of when I had written down, so I wouldn't worry too much, it's one of those things that really can't be controlled all that much without super accurate weather forcasts and perfect instruments.

And about climbing the couple of thousand feet, I meant if you started at sea level and climbed up to 12,000 feet, then airspeed would really change, but I figured you wouldn't be doing that in a 1975 Cessna 150, so I said it probably wouldn't change that much.  And winds can really change, even in a matter of 500 feet, so you never know, there could have been a change in wind direction or wind speed between 4,500 and 5,000 feet, that just one of those things you can't really predict all too well, and you just have to account for it while you're flying.

Just my 2 cents.

No problem, as long as your not flying to a checkpoint that's five miles away, a minute should be a pretty good time.  That's my opinion at least, others may feel differently though.

Checkpoints ranged from 10-15 nm.

Hmmm...
My guess is the same as yours: that you miscalculated your TAS. Winds figure into it, of course, but it sounds like you're not sure where you get TAS from, or what you should have indicated on your ASI and your tachometer for your target TAS.
I use a whiz-wheel, so I don't know where you went wrong, but I do know that TAS is IAS corrected for pitot-static installation(which is hardly a factor at such altitudes and speeds),pressure altitude and temperature. Humidity doesn't figure into it very much, either,apparently... temperature is what makes the biggest difference in density for a given pressure altitude.
  You should also fudge the book numbers a bit, unless that plane just finished its break-in period right out of the factory... those old trainers are always slower than the book. Always. Hell, they were probably not as fast as the book says when they were new.

For what it's worth, my 1968 C150 (Continental O200A) Pilot's Operating Handbook shows that the "sweet spot" (closest margin between fuel burn and airspeed) for 5000 feet at 2600 RPM, which should yield 113 MPH TAS at 5.3 gph at standard temps for that altitude. That's 98 knots.
That old 150 I used to fly did not quite deliver as advertised... I usually planned (at altitudes up to about 5000) for 90 kts indicated; for typical flying in good weather , winter or summer, that would get me 95 to maybe (maybe!) 99 knots TAS at 5000. Just shy of the book, generally.  Never worried about a minute on checkpoints during my training (you can't constantly re-calculate GS on such short legs- the air is unruly), unless of course I got so far off-course that I was wandering around looking for it...