Non ADS-B. Mode S only aircraft

My own radar is picking up plenty of Mode S only traffic, and sometime these are displayed on the fr24 map originating from an MLAT source which i understand.
What i don't understand is sometimes this Mode S traffic shows as originating from a T-xxxxxx feeder, which i had assumed is using the same technology as Rpi implementation.

Can someone be kind enough and explain therefore how t-xxxxxx feeder can show as obtaining a location fix on an aircraft that is only transmitting Mode S ? Is this an Fr24 anomaly or are they getting the GPS co-ordinates from another source ?

Thanks