The base comparison price for a configurable product seems to be the cheapest price excluding tax. Then the extra price added by the each configurable product in a Magento 2.3 dropdown is calculated including tax. This makes even the cheapest option appear as it has an extra cost, which is only the tax.
See the screenshot below where the actual price of option "Test 1" is $62.50, but option "Test 1" still appears to be $12.50 more expensive than the cheapest/base alternative. the +$12.50 shouldn't be there, since its already the cheapest alternative.