This topic might have been put to bed, but for the record, it looks to me like jnh is right - the girlfriend has been refunded the right amount and you've been overpaid.
The TRS available to each of you is the lower of 20% of your share of the interest paid, or 20% of the ceiling of €4,000.
The total interest paid was €4,203.11 therefore each of you paid €2,101.56. As this is below the ceiling, you each should have got a refund of 20% of €2,101.56 = €420.31.
Which is what your girlfriend got.
Revenue refunded you 20% of the ceiling, i.e. €800, and this is the incorrect figure - you should check that you didn't inadvertently include the wrong figure yourself on your form.