Schema requires gather stats even after automatic gather status is SUCCEEDED

hi,

A few questions about the automatic gather stats operation in oracle

1)

Anyone knows why oracle gather statistics schedule is automatically setup to run for 20 hours on weekends and 8 hours on weekdays?

2)

If threshold for gathering stats is 10% and somehow oracle does not have the chance to gather those stats it'll postpone it to next day or to the weekend?

I ask because I observed sever schema slow down which also affecting other schema. This gets resolved once I run gather stats on that particular schema so I'm thinking maybe the optimizer is postponing things till the weekend

exec dbms_stats.gather_schema_stats(ownname => 'XYZ_OWNER', options => 'gather', no_invalidate => false);