The 13mm refers to a measurement of the thickness of the rod in between the writsping and crank ends. The 'low friction' skinny rods are much mroe slender, and weigh less as a result. In theory making the engine spin up a little quicker, use a little less gas.

These were used from 85 - 89. So a 90+ motor will have 13mm thick rods, an 89 will (most likely) have skinny 9mm rods.

Not a big deal unless you turbocharge and run some HP through them, the 9mm rods are on borrowed time, and any pinging/detonation/tuning erros and they'll likely go quickly. 13mm rods will stand up to a lot more knuckleheaded abuse.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)