The level of a mob is calculated based on the parameters of each mob. This includes things like Max HP, Regeneration, Attack speed, Accuracy, Damage, Range, Speed etc, they don't all weigh as heavily in the calculation however. High regeneration adds a lot to the mobs level and used to add even more, until a patch about 1.5 years ago or so as some might remember where the levels of many mobs changed slightly towards higher levels on high damage creatures rather than high HP/regen creatures.
In the case of Falxangius vs Mulak'f it's partly based on the large regeneration difference with very high on Mulak'f to very low on Falxangius. There is however a big difference that isn't as easy to see between these mobs which is their attack speeds and accuracy. The Falxangius attacks quite a bit slower than Mulak'f and misses a lot more giving it perhaps a lower overall damage output (depending on evade levels).
However it is true that mobs that are generally considered more dangerous are the ones that hit hard enough to kill you in one or two hits. It's also harder to use armor to deal with such mobs while a monster like Aurli seems pretty easy because it hits super fast but not very hard making it easy to get an armor to absorb pretty much all the damage even though it might actually have higher DPS then a mob that feels tougher because it hits hard and with concentrated damage like the Falxangius.
Regarding mission rewards, they are not based on the mobs levels but another calculation. It usually correlates quite well with the mob levels anyway though.