Why wasn't I issued a new I-94 after previously having had returned one at the Canadian border?

A couple of weeks ago I traveled to the US by car and received an I-94 for 3 weeks. Then 4 days later I've returned to Canada and handed over my I-94 to the CBSA officer. The exit was properly recorded and I can see it on the DHS website. Today I've traveled to the US by car again expecting to receive a fresh I-94, but instead the border guard let me in on my original one. This forced me to re-print my I-94 before I went back as otherwise I wouldn't have been able to give one to CBSA on my way home.

So what are the rules for receiving a fresh I-94? Does it never expire before it's original expiration date even if DHS knows you've left the country?