My observation is that companies often aren't clear with themselves about what they want. What's in the contract is one thing - but then project phases come up, new managers, or "we need more collaboration" and suddenly it's supposed to be three or four days. Especially at the beginning it tends to creep up, because you "still need to get up to speed" and should be present.
The problem: you often don't realize it until you're already there. Some companies are honest about it and tell you early if things get more flexible, others just slip it in on you. If the position matters to you, I'd ask specifically how it actually looked for them over the last two years - not what the HR person says in the interview, but how it really played out. If you know someone who works there, ask them.
Realistic rule of thumb: count on at least one day more than agreed. If 2 days are contractual, plan for yourself with 3. That way you won't be caught off guard and can sign with peace of mind.