You are right - but though there are many differences between the individual Celtic languages and the two major groups, they do show many family resemblances. For example, no infinitives, two grammatical genders,
a vigesimal number system (counting by twenties)
e.g. Cornish hwetek ha dew ugens "fifty-six" (literally "sixteen and two twenty")
verb–subject–object word orders
an interplay between the subjunctive, future, imperfect, and habitual, to the point that some tenses and moods have kick the others out,
impersonal or autonomous verb forms serving as a passive or intransitive
Welsh dysgaf "I teach" vs. dysgir "is taught, one teaches", Irish "déanaim" "I do/make" vs. "déantar" "is done"
So, I guess it's not surprising that there is crossover and commonality in word usage.