New try on RE...
Level - Poor/Medium/Fast Defense Progression
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1st - +0/+1/+2
2nd - +1/+2/+3
3rd - +2/+3/+4
4th - +3/+4/+5
5th - +4/+5/+6
Now... let's see if it can be the case...
Death Star Trooper
nonheroic 3 - ??? (mabe nonheroic character has no progression of Defense?)
soldier 1 - reflex fast, fortitude fast, will poor
defenses striped-off ability bonuses and "take 11"
Ref 2 (+0 non, +2 soldier) Check
Fort 2 (+0 non, +2 soldier) Check
Will 0 (+0 non, +0 soldier) Check
Chalkazza
scout 1st - probably fast ref, mediocre fort, poor will
Ref 2 (+2 scout) Check
Fort 1 (+1 scout) Check
Will 0 (+0 scout) Check
Jensaarai Defender
Jedi 5/soldier 3
soldier 3 - reflex fast, fortitude fast, will poor
Jedi 5 - reflex fast, fortitude medium, will fast
Ref 12 (+4 soldier; +6 Jedi) not Check (strange? Armor?)
Fort 9 (+4 soldier; +5 Jedi) Check
Will 8 (+2 soldier; +6 Jedi) Check
Han Solo
Scoundrel 5 ref fast, fort and will medium
Soldier 2 ref and fort fast, will medium
Ace Pilot 1 ref and will fast, fort poor (just guessing)
Gunslinger 1 ref medium, fort and will fast (just guessing)
Ref 12 (+3 soldier; +6 scoundrel; +2 ace, +1 gun) Check
Fort 10 (+3 soldier; +5 scoundrel; +0 ace +2 gun) Check
Will 10 (+1 soldier; +5 scoundrel; +2 ace, +2 gun) Check
In this last one, lot's of things might be slightly different, but the whole will stay the same. Anyway, even if I am a bit incorrect (which is very probable) you can see overall speed of Defense progression, which is dramatical on way or another.
Although this (or similar) variant seems probable, it does not explain everything... the strangely high Jensaarai Defender Reflex, possibility or not counting other factors elsewhere and especially defence in nonheroic characters!