Here are my thoughts:
There are differences in power output in the upright road position and the aero TT position. This could quite easily be the difference between your first and second FTP tests (238 vs 220 is less than 8%).
There are also differences between indoor and outdoor performances due to cooling and pedalling dynamics.
These two factors could make your indoor FTP in the upright position very close to your outdoor FTP in the aero position.
Holding power above FTP for 20 minutes shouldn’t really be taken as a sign that your FTP is too low. FTP is power that you can hold for anywhere between 30 and 70 minutes, depending on your physiology and how well trained you are.
If the 20 minute average is not preceded with a 5 minute all out effort the result is not valid for applying the 0.95 correction factor. Even if there was a max 5 minute effort the 0.95 x 249 gives an FTP of (as you stated yourself) 236, so how you think it’s too low at 238 is beyond me.
Having said all of that, you’re talking about differences of 2 Watts - it really won’t make a difference to your training but don’t get sucked into thinking that you need to try to justify the highest FTP you can.
If you’re on the trainer in the TT position, you may be better off using 220. If you really want to know if the ramp test result if a fair reflection of your abilities, do The FTP challenge.
Mike