OK I admit I'm an idiot when it comes to this stuff. I borrowed a compression tester and checked the compression of each cylinder. Three were in the 70 psi range and one was 80. Is this right? Shouldn't it be a lot higher than that? Am I doing something wrong?

 

Look at the video in the link below prepared by rspi which should help you along.
He has hundreds of various how to do videos that are very useful guides for Volvo owners especially.

Your readings are too low but if one does a 'wet test' they normally show higher.
Wet test involves a small amount of oil being put into each cylinder prior to test.
i.e. about a teaspoon per cylinder.
A positive sign is that your readings are all even but 1 and thats only 10psi away which is normally quite acceptable.

http://www.matthewsvolvosite.com/for...ic.php?t=57194

 

70-80 is low... How many miles on the car? What ails it?

 

70-80 = no start.

 

What prompted you to do the compression test originally?

If the vehicle has not been started for a longish period it may have lost compression due to what is known as 'lawnmower syndrome'.
(lack of cylinder lubrication results in the piston rings becoming unsealed.)
Adding small amount of oil into each cylinder helps to lubricate, reseal and increase compressions sufficient to start up.