Can't get my head around this possessive apostrophe

You are right, but most people would leave them out...I suppose because "compared to your peers" in this case means compared to your peers' salaries.
It is understood what you are comparing.
But you are technically correct.

Many thanks. I would prefer to leave them out but don't want to annoy the grammarians.

Apostrophe Usage Chart

Ownership
To indicate ownership by a proper single noun.

For example:
Susan owns a hat. It is Susan’s hat.
Barry owns a car. The car is Barry’s.
Agnes owns a restaurant. This would be Agnes’s restaurant.
To indicate ownership by a proper plural noun.

For example:
The Lin family owns that lawn mower. It is the Lins’ lawn mower.
The Edwards family owns a painting by Picasso. The painting is the Edwardses’.
Mary-Kate and Ashley Olsen own a mansion together. This would be Mary-Kate and Ashley’s mansion.
Contractions
To indicate one or more missing letters in a word.

For example:
She is not at work today [becomes] She isn’t at work today.
I’m afraid I will not be able to attend the party [becomes] I’m afraid I won’t be able to attend the party.
Because she does not like malls, Jennifer is doing all her Christmas shopping online this year [becomes] Because she doesn’t like malls, Jennifer’s doing all her Christmas shopping online this year.
Don’t Make These Mistakes!
If you’re talking about something that belongs to an “it,” you do not use an apostrophe.

For example:
That tree has pretty leaves. Its leaves are pretty.
We need to fix the legs on that table. We need to fix its legs.
My favorite store’s jewelry is all on sale. Its jewelry is on sale.
In general, apostrophes do not belong in plural words.

For example:
I have a pile of paper’s to grade. (Incorrect)
I have a pile of papers to grade. (Correct)

Do you want to use vanilla or chocolate frosting for those cupcake’s? (Incorrect)
Do you want to use vanilla or chocolate frosting for those cupcakes? (Correct)

Look at those airplane’s over there! (Incorrect)
Look at those airplanes over there! (Correct)

Do not use apostrophes when creating plural acronyms and years.

For example:
There are three ATM’s at the bank. (Incorrect)
There are three ATMs at the bank. (Correct)

The only people allowed inside are the VIP’s. (Incorrect)
The only people allowed inside are the VIPs. (Correct)

My mother was born in the 1950’s. (Incorrect)
My mother was born in the 1950s. (Correct)

If a word end in s, you don't have to add another s, just an apostrophe will suffice. The above examples are correct, but so are these.

This would be Agnes’ restaurant.
The painting is the Edwards'

That wasn't what the OP was asking about.
Read it again.

'With this Report you'll see immediately how your pay compares to your
peers"

or you could write

"With this Report you'll see immediately how your pay compares to that of
your peers"