Depends on decay setting and how often you force BT events if they aren’t occurring while training indoors or riding outdoors.
During Base phase many users change to No Decay as they don’t expect to be testing themselves with any BT efforts. Any change to signature will be gradual and TL based (up as you increase training load or down if you take days off). However, with optimal decay (what I used for entire progression) your signature will trend downwards without BT events but TL is factored into the calc. So the graph may look fairly level while load increases until you test yourself again to validate any signature changes.
That’s what is going on here where 3 weeks or so into Base I purposely tested myself with a BT workout. A BT workout won’t show up in recommendations. You make that decision when you are feeling fresh and want to check on progress. In my case I achieved a Gold BT riding Under Pressure (revised) which is a BT workout I selected from the library. My next circle is a bronze BT from a workout on the recommend list during Build. Then I tested again shortly afterwards for my next gold (using same BT workout which is my favorite).
As you recover from recommended training load and feel stronger you may want to test more. You may also encounter BT opportunities during HI workouts from the recommended list. On these occasions if feeling good you can change to slope mode and go for broke with the goal of exceeding target watts and interval durations to force a BT. I didn’t try doing that on my first full progression because it was an experiment, and the HI workouts were hard enough during Build/Peak that I didn’t want to jinx my 100% at 100% goal. 
As far as the predictions go those are relative to where you are now and what if you can achieve (mathematically) if you are able to recover from the projected training load increase over time. What will happen IRL is the rate of improvement will likely slow as you get closer to your achievable fitness level for the training block. XATA in turn will revise the numbers.
The prediction I refer to in this related thread was what was shown during Peak phase. I didn’t monitor prediction changes that occurred from when I started the progression. –
How accurate are the TP predictions for the training programs? I think mine is way off - General - Xert Community Forum (xertonline.com)
As far as year over year gains go, I guess I’ll know for sure by July 2022.
Currently I just started a new progression, but at a much higher TP than last December. I will have to blame Xert for that. 
I am not expecting similar results this time around.
BTW: Some experienced endurance riders who specialize in long events prefer No Decay or Small Decay instead of Optimal because they purposely don’t challenge themselves with HI during their training. This assumes signature is dialed in to start with (BT tested) and you continuously train without any major gaps. At some point though you will want to test your fitness levels to confirm status.
Reference –
What do the coloured circles on my XPMC mean? – Xert (baronbiosys.com)
Xert’s Magic Setting – Signature Decay – Xert (baronbiosys.com)