Signature campaign: The lowest and highest payment BTC and USD in 2013-2022
I don't know what you refer to exactly when you wrote "their pay rates to users are different", probably to the higher/lower payments per campaign, or per user group (Legendary, Hero etc.). But the payment is the same regardless of the geographic location of the user (which is unknown in most cases).
So with signature campaigns we have one of these rare cases where a payment for a service is already "balanced across the world".
This however has probably some consequences to the type of users who write for signature campaigns. While in some less developed countries a signature campaign payment may be already similar to the income of a regular job, in the US or Europe it probably is an additional income, or done by students for example. This is not so different for other simpler freelance work open for international applicants, like simple programming tasks to help IT students, SEO texts and so on.
The "balancing" effect was actually even more pronounced in the past when most signature campaigns even calculated their payment in BTC or other coins, and not in USD.