Both TOPS numbers can be true at the same time.
A model may use the 26 TOPS or very close to it at a specific instance in time when all compute units are active. However because every layer requires different amounts of compute they run at different FPS. Therefore layers with higher FPS will have to wait for the layer before or after. During these times the layers are inactive resulting in a lower average TOPS number.
For realistic models it is not possible to make all layers run at the exact same speed and therefore the average TOPS number will be lower than the peak TOPS number.