New Game with settings below. Use FOW to reveal map, no elerium to be found. Repeated multiple times.
Nebula: Occasional
Resource Freq: Uncommon
Change Resource Freq to Occasional: Suddenly plenty of Elerium.
Looking at MapSetupDefs.xml, Uncommon vs Occasional settings. (Yes, I know that Elerium is a nebula based item)
Uncommon:<MinableAsteroidChance>0.4</MinableAsteroidChance>
Occasional:<MinableAsteroidChance>0.6</MinableAsteroidChance>
So, the pattern is that it increases, same with pirate bases, rifts, anomalies and black holes even Nebula rate. All decimal values less than 1. (Admittedly I didn't look at every setting for every frequency).
Oddly then for each of the Frequency options we see this.
None: <MinableNebulaRate>100</MinableNebulaRate>
Rare: <MinableNebulaRate>30</MinableNebulaRate>
Uncommon: <MinableNebulaRate>25</MinableNebulaRate>
Occassional: <MinableNebulaRate>20</MinableNebulaRate>
Common: <MinableNebulaRate>15</MinableNebulaRate>
Abundant:<MinableNebulaRate>10</MinableNebulaRate>
Looking at them in total. It seems that MinableNebula is simply defined differently for some reason.
Comparing the entire Occasional vs Uncommon definition the only differences beyond the obvious numerical changes are in some of the formatting and how less than one values are entered. Example: <Rifts>00.20</Rifts> vs. <Rifts>0.03</Rifts>
For the sake of testing, I changed the MinableNebulaRate in Uncommon to 20 from 25 and a majority of the Nebula now have Elerium vs None with a setting of 25.
All in all it seems that there is an issue here.