the difference between yeshiva and sem is that the girls are only there for one year, maximum two depending on where they go whereas most bochurim are in yeshiva for anything between two to five years before they start shidduchim. Think about it – in England, most of the girls are only 16 when they go to sem but by the time they are finished they are old enough to start looking.
Also it depends on the maturity of the girl, and whether they are actually dating or if something ‘comes up’. Most of the girls who get engaged whilst still in sem were shidduchim that came up and worked out. However, a girl should not be actively ‘on the market’ when she is in sem as it will disrupt her sem education.