Why are language test scores requirements sometimes higher for undergraduate than graduate admission? I’ve been searching for the graduate requirements of some university. As regards the language test scores, TOEFL>90 or IELTS>7 are necessary. However, I stumbled upon undergraduate requirements: TOEFL>100 (>25 each) or IELTS>7.5.

In my opinion, graduate studies are more complex thus require higher levels/skills, so what could be reasons that the graduate scores are lower than the undergraduate ones?
While I don't know what exactly went into these decision making processes, here are some relevant factors (some more likely than others):


As mentioned in the comments, undergraduates need to take a wide range of classes (e.g., gen eds) including classes which may involve a serious amount of reading, writing and/or verbal interactions.  So undergraduates need pretty well-rounded English skills.
For graduate studies, one can often get by with a much more narrow range of English proficiency depending on the field.  For instance, less language skills are needed to follow a mathematical lecture than one about politics.  My school has higher TOEFL requirements for certain graduate programs (e.g., business administration and political science) than the general grad college requirement.  A more flexible policy makes sense for grad programs.
While some grad students may have teaching duties, there can be separate English proficiency requirements for this.  My school does this, and international grad students just grade rather than teach until the English requirements are met.
By allowing grad programs lower standards of English, one can get a larger pool of international grad student applications.  (Both increasing the fees obtained and improving one's chances of getting academically stronger international grad students.  If we really want a specific student with poor TOEFL scores, we can petition the grad college to waive the language requirement for them, but if the student just looks at the online requirements, they may not apply in the first place.)
Having higher English requirements for undergrads may make a school seem more selective (this may be factored into college rankings, but the grad student requirements may not be).  Related to this, in the current age colleges are evaluated by student success rates (e.g., pass/fail,  retention and graduation rates).  Colleges may feel/have evidence that international students with lower TOEFL scores have lower success rates, and not want to take chances on them.
Higher TOEFL requirements may boost enrollments in the university's ESL program if there is one.