Hello Robert,
The subscription which you purchased from Google play store can be installed only on 5 android devices.
Could you please confirm whether you need to activate this subscription on your Wife's android device?
Also, please share us the screenshot of the page which shows "Subscription can be used up to 10 devices". So, we will check and help you. Keep us updated. Thanks.
I have just paid my yearly subscription for AVG Ultimate for £39.99 (which I not cheap by the way!) through Google Play and it says I should be able to use it on up 10 devices, however, after trawling the AVG website and googling it, I can find no way to share my AVG subscription with my wife. Any help appreciate. 
Hello Robert,
The subscription which you purchased from Google play store can be installed only on 5 android devices.
Could you please confirm whether you need to activate this subscription on your Wife's android device?
Also, please share us the screenshot of the page which shows "Subscription can be used up to 10 devices". So, we will check and help you. Keep us updated. Thanks.