question on binding data and primary key for a table

Suppose I have a table my tab with primary key(index is mytab_pk). I have 2 named caches: My default Cache, My Table Cache.

my db is bound to My default cache

my table is bound to My Table Cache.

Then I bind my table to My table cache: the data is bound to My table cache, and pk is bound to My default Cache automatically.

This is okay.

Then I try to bind mytab_pk to My Table Cache too with sp_bindcache. then I get very long time wait for same data operation.

Not sure why. So binding for index on pk should not change its default way of binding?