technically, its a 100% hit.
according to the SC, if it gives a 20 bonus and ur stats show up its 163, it shud hit at 100% chance !!
so, try testing again, check ur stats, are u missing a buff or something .. may have missed a blessing or something. Also, try to use a +1 or +2 dex food and check if u hit at 100% chance. If that works, just get 1 dex stat foods, easily save up on ur stats !