Hmm I would think it’s actually for the reason they give which is higher bills than expected from Vodafone. (Based on claims that 1&1 used a higher % of their network - as Vodafone’s own data usage grew more slowly than expected)
I hope this will stay a one-off adjustment in this regard and would not lead to consistently higher bills from Vodafone than 1&1 expects or other strains in the partnership.
My main reason to believe that they have indeed been surprised by the bill is that they needed to adjust their yearly guidance which was confirmed as recently as Q1 2025.
So if it would have been mainly due to other factors they could have adjusted before.