Strange or perfectly logical?

Today I was at Cardiff Central awaiting the 1855 Manchester Piccadilly train.When it arrived on time from the west it was terminated and sent back to Carmarthen.
Meanwhile the Manchester To Carmarthen service was terminated at Cardiff and sent back to Manchester as the 1855 service about ten minutes late.
Both trains were three coach 175s.This is the second time I have had this happen in recent months.What could possibly be the reason for doing this?

Furryanimal said:

Today I was at Cardiff Central awaiting the 1855 Manchester Piccadilly train.When it arrived on time from the west it was terminated and sent back to Carmarthen.
Meanwhile the Manchester To Carmarthen service was terminated at Cardiff and sent back to Manchester as the 1855 service about ten minutes late.
Both trains were three coach 175s.This is the second time I have had this happen in recent months.What could possibly be the reason for doing this?

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More than likely one set required to finish in a particular location for maintenance reasons.

The 1855 forms a Chester service once it reaches Manchester, and then stables in Chester Depot. That will almost certainly be the reason, given that Chester is the main depot for the 175 fleet. So perfectly logical is the answer to the original question.

Naturally control will try to plan things so disruptive changeovers aren’t necessary, but this isn’t always possible, especially if something unforeseen happens (for example unit develops a fault whilst on the road, or service disruption takes a unit off the diagram which took it to where it needed to finish, or they couldn’t get it on the right diagram in the first place for whatever reason, etc etc).