The speed can only be controlled by one thing at a time, either the pitch or the thrust.
If the pitch is flying a straight line such as VNAV PTH (or ALT, or VS, or GS) then the thrust is controlling the speed, in SPD.
If, instead, the pitch is controlling the speed (VNAV SPD or MCP SPD) then the thrust can not be also controlling the speed. It is fixed, either at idle or a climb setting.