Would be interesting to know if both aircraft were flying 15a planned TAS, or otherwise.
Anything with an FMC and running on Cost Index will not be maintaining a constant planned TAS - actual TAS will change dynamically based on wind, & weight throughout the flight. With ADS-B and/or CPDLC TAATS has real-time access to the TAS. If TAATS does not use this information then a problem exists at that end.