Transformer Output Ratio not Exact?
Transformers are usually rated (volts in / volts out) based upon full load current. You are measuring during no load.

So you measured the no load (unloaded) voltage is 17.5 v = 122/6.97 v.

The Un-loaded voltage is a more accurate measurement of the turns ratio.

Fully loaded, you will get lower voltage than 17.5 volts, which results in your transformer's rating of 15 volts output. 

Fully loaded means that there will be a voltage drop on the internal resistance of the transformer,