@bfrostie I know you are pointing out a discrepancy issue when using another testing protocol, but for those who may stumble onto this thread in the future here are some key points –
Consistency is crucial to successful training and that also goes for testing protocols.
Each platform has their preferred testing method which works well for the majority who train on that platform. However, the same test will fail a minority who don’t respond well to that type test or simply fall on either end of the bell curve. Plus test results from one platform don’t necessarily translate to another.
For the most accurate results with Xert you’ll want to test the Xert way. 
Breaking Through the Xert Way! – Xert (baronbiosys.com)
Once every 3-4 weeks – if your regular ride/workout activities aren’t generating BT events on their own – select a BT workout from the library and ride it in slope mode. You can be “said and done” in as little as 10 minutes worth of HI efforts. You’ll never need to do a RAMP, 20 minute, or 8x2 test ever again. 
Of course you can do them if you’d like to but those tests may not provide sufficient data points for Xert’s signature algorithm to work as designed.
Consider your PP value can also rise without actually hitting a higher number on your power meter. For example, some BT workouts include a long interval where you drive MPA down then stand up (right before failure), spin up, and sprint as hard as you can for at least 5 seconds (while fatigued). The measured power will be considerably lower than a sprint effort while fresh but it’s enough for the Xert algorithm to predict PP. If prediction is higher than current value you’ll get a bump in PP. You don’t need to be a sprinter or practice sprinting to accomplish this, but it helps to practice BT workouts to understand how they work and determine which one(s) are your favorite to do.
It is also common for your sprint power indoors (power meter value) to be considerably lower than what you can accomplish outdoors. A jump of 150-200 watts outdoors isn’t unusual.
Everyone has sprint power whether they have reason to use it on occasion or not.
Think what would happen if you found yourself between a grizzly bear and their cub crossing a road. The mother bear may outsprint you but at least you tried. 