Then it's a true mystery if they were both at -42 m.
But they weren't, were they? According to the OP:
Relative Altitude : +13.50
Absolute Altitude : -42.00
That was the reason for his question; he took off from ~ 0 m MSL, and at a barometric altitude of 13.5 m he found - 42 m MSL from GPS in the metadata.