There are several topics discussing what you describe. Here’s one –
100% TP for more than an hour in workout creator? - General - Xert Community Forum
TL:DR – I wouldn’t worry too much about the gap in the model below TP. 
Xert is aware plus you are never going to train that way. While you might include SS blocks for “harder” endurance training the majority of users opt for recommended LTP/Z2 level strain on LIT days.
Including Tempo/SS in endurance level workouts boosts XSS per hour and the Difficulty Rating (Score). You can see this visually in Workout Designer.
For example, compare these three 2-hour endurance level workouts (no MPA draw down).
88 XSS, Easy 1-diamond Difficulty (43) –
107 XSS, Moderate 2-diamond Difficulty (55) –
145 XSS, Difficult 3-diamond Difficulty (75) –
On my LIT days I’m picking blue/aqua (~LTP) most of the time (not green tempo/SS). YMMV