"To help identify and prevent fraud, information about how you use your device, including the approximate number of phone calls or emails you send and receive, will be used to compute a device trust score when you attempt a purchase," the page reads.

See the "information about how you use your device, including the approximate number of phone calls or emails you send and receive".

So the score calculation for that device includes the approximate number of phone calls or emails you send and receive on it.

You could interpret that as "the more personal activity you have on a device, the less trusted it should be", but that makes no sense. The more you use a device, the more likely it is to actually be your device.

It makes sense to place less trust in devices that you rarely use.