Back when film speed was determined by the ASA (American Standards Association) a specific developer was specified. The formula for which was part of the ASA standard as it was not a commercial developer. Now with ISO standard the manufacturer is allowed to pick the developer that they wish to use. Different manufacturers can use different developers. I personally have never found this idea to be particularly attractive. As the old saying goes "The nice thing about standards is that there are so many of them."
Certain developers like D-76 and Rodinal (with other analogs like R09) do not produce full rated speed. So its is not surprising that Fomapan 400 only yields an EI of 250 in Rodinal. Manufacturers design their films and developers together. My advice would then be to look into one of Foma's recommended developers like Hydrofen. I published the formula and developing times on APUG some time ago.