I've seen a couple of people get confused over this, but it's simple and makes sense once you realise what it's doing
It's +100%, not x2
Look at your current exp rate, it's already got other boosts such as subscription, resting, 2nd char, tao buff etc. It's not uncommon to see exp rates at 500%+
So if your exp rate is say 450% already due to other buffs, then pop a 100% pot it will become 550%
This is why I generally recommend that spending on pots like that is only worth it when it heroes league because most other buffs are gone, so you get better value out of them. Increasing from 130% to 230% is a much better step up than from 450% to 550%
You don't really need to worry about them though, levelling is so fast these days anyway it barely matters. Spend your gg on improving your kit instead, being able to kill faster and having to go home to re-pot less often will be more beneficial to your levelling time than a nudge more exp per mob