if you have a narrower grip with your palms towards you then you will probably use your bicep a fair bit to help, especially if you have trouble with lat activation / have weak lats.
if you widen the grip and have your palms facing away from you then you position yourself so your biceps can't help as much, which really focuses on the lats needing to do the work.
the former is a chin-up in the vertical plane, the latter a pull-up. pull-ups are harder. much much harder for those who are having trouble with their lats. don't be afraid to use the lat pull down machine or a lot of assistance to perfect your form (lots of articles on this site on pull-up / chin-up form).
do you do any pulling in the horizontal plane? inverse / inverted rows or anything like that? same thing goes...