Okay, I'll simplify. Store advertises three sandwich. You buy three sandwich. You get one sandwich. Store says fees+taxes ate other sandwiches. You say it's fine, you got sandwich. I say it's not, store lied.

They absolutely can give a solid number even when a lottery runs in areas with different taxes, they simply choose not to because they make more money that way and for some reason you lack regulation there. See for instance here where the prize money may be partially subjected to income tax, meaning tax varies wildly depending on the winner's other income:

£10,000 every month for 30 years. [..] However, based on tax rules and rates at the date of these Procedures, the monthly payments will not be less than £10,000 after tax.

So there's three obvious choices: mislead customers, calculate the correct prize after relevant taxes and advertise that, or give a fixed value and eat the cost of any taxes themselves. They chose the first one.