That's because the selection of numbers is a little bit weird. The 70 billion on the cost side are composed of 38 billion for construction and maintenance, 14 billion for traffic police and 18 billion for public funds spent for accidents. The generated income is only taxes on fuel and the tax car owners have to pay.
If you include the cost of the traffic police, there is way more stuff that you can include on the income side like taxes on car sales and part of the cost comes also back to the government in the form of taxes. There is likely also a large part of the costs that is missing. Doing this properly is a lot of work and doing it precisely is hard to impossible. These sort of things almost always include estimates for the higher order effects.
Btw: I googled the study[1] and apparently it was funded by the "Netzwerk Europäischer Eisenbahnen e.V." (Network of European Railways Association). I would take any statements and numbers with a huge grain of salt.
Maybe in UK local roads are funded differently? If you counted only national roads in Poland it would seem that Poland takes more in revenue than spends on roads, which isn't true if you count expenses on all the local roads that aren't in national budget.
