CONFUSING QUESTION ABOUT COLLEGE ADMISSIONS?

I do realize that colleges want stuff in before first semester of senior year, but if colleges also look at senior year, why is junior year so important? And it also doesn't make sense to me how if someone's strengths lie in senior year (ex. 4.0 GPA, 3.8 cumulative, 5-6 AP classes, tons of ECs, community service) but they did fairly or satisfactorily in junior year (3.2 GPA, 3.7 cumulative, 2 AP classes, fair ECs) and they get rejected from a college. Do colleges ever reconsider (admissions-wise) someone if they did amazing senior year with the stats above, if they got rejected before with the stats above for junior year?