Yes, this is the explanation.
Eisiau (isio) should really be used as a something that isn't a verb, and take the form "Oes eisiau cwpan arnat ti" (Is there a want / need for a cup on you), but in the south we use moyn, and eisiau = a need, and in the north, they tend to use isio as a sort of verb, though it shows itself up by not following ver rules with yn etc.
However (that explanation having been more confusing than enlightening),. this leads to odd patterns in answering, like "Wyt ti isio oren?" "Oes". Its part of what makes a second language so much more interesting.
BTW, rememeber that want / need are classed as temporary things, like certain illnesses, so they are "on you" in Welsh, rather than "with you" - "Mae isie rhywbeth arna'i".