Hi,
I've been using FT8 for some years, but since the last to months i've been enjoying FT4.

When using FT4 mode right in the middle of a QSO with a station, I sometime receive a QSO call (addressed to my callsign) from another person on another offset frequency.
I never see this happening when using FT8, only in FT4 mode. It appens on the 20m and 40m bands. 

Is this 'technique' use for being more efficient, meaning that after completing the 1st QSO, I dont need to call CQ again, and just reply to the 2nd station (using is calling offset frequency) ?

Thank you, 73
Sebastien , VA2SIB