I am learning the Mishnayot of Masechet Nedarim and I came across an apparent contradiction that I cannot quite wrap my head around. The seifa of the first mishna in the fourth perek mentions that a person (we'll call him Reuven) who has made a neder not to benefit from his neighbor (Shimon)'s food cannot borrow even a non-food-related tool for free if it is the type of tool which would normally be rented. The Rav Bartenura explains based on the Gemara that the reason is because Reuven could use the money that he saves by not paying to rent the item to buy food and he is therefore benefiting food-wise from Shimon.
Fine. So far I understand.
Now, the next couple of mishnayot discuss the kinds of services that Shimon can do for Reuven if Reuven made a neder not to get any הנאה from Shimon at all. Among these things are certain services that definitely save Reuven money. For example, Shimon may pay Reuven's מחצית השקל (since the money goes to the גזבר rather than to Reuven), or he may settle Reuven's debts (similar reasoning), or he may provide food for Reuven's wife (similar reasoning).
My question is, what's the difference between the first case where Reuven is not allowed to indirectly get הנאה from Shimon and the second case where (at least to me) it seems like the הנאה is much more direct, but is still permitted?