Water filter rated for 6 months or 1500 gallons - which is it?

I have a 3M undersink water filter (3US-MAX-S01). It's rated for 1500 gallons, but the box also saids 6 months. It's mounted only on the water filter spigot, so we use at most 3 gallons per day (family of 4). If I go by the gallon rating, a filter should last us over a year.

But am I missing something? Is the 6 month rating take precedence over the 1500 gallon rating?