Why is there a difference between the BAT I receive on my phone and my desktop?
Advertisers may choose to target specific platforms so you are never guaranteed to receive the same number of ads between two or more different devices.
Certain choices made by advertisers also affect the amount of BAT users receive per ad, so even if two devices receive the exact same number of ads they may receive significantly different amounts of BAT.
You can view the number of ads available in your region by platform here: https://www.kenherbert.dev/brave-ad-stats/.
As long as your estimated pending rewards are within acceptable limits then some difference in amounts earned and ads viewed between two different devices is expected.