The low numbers always win. A cat can start out high but the numbers then can drop to a low point. The peak or nadir is what Lantus dosing is based on. If you consider the big picture it will (hopefully) be clearer.
Here's an example: Gabby at one point had an AMPS in the 400s. Her PMPS was also in the 400s. Just looking at those two numbers, you would think a dose increase was needed. However, her low for the day was in the 40s. She got a dose reduction. This also illustrates why getting those spot checks is so important.
If you were to increase based on the high numbers, you could easily put your cat over dose and end up working very hard to get numbers out of the basement. It would be dangerous. While there are a few instances when you might not decrease the dose when you get a number under 80, the majority of the time you decrease the dose.