Your math fails to tell the whole story. Of course the number of Al Nusra fighters is substantially lower than that of ISIS. The estimated strength of ISIS ranges from about 30,000 to over 100,000. Most estimates for the strength of Al Nusra around 5,000-10,000. They also hold substantially less territory and are imbedded in areas that are divided between multiple different groups. The only place that they have under their control is in northwestern Syria near Idlib. ISIS has essentially the whole central and eastern part of the country under its control as well as large parts of the west.
Their operations are very different. ISIS is intent on creating a state and has to operate somewhat openly and moves about in convoys, carries out more conventional assaults on targets, rather than focusing on guerrilla tactics. They have more central organization, while Al Nusra operates as various cells. They have the assets (tanks, humvees, etc. they stole from Iraq) to operate as a more traditional fighting force. Al Nusra doesn't have that ability. They operate like AQ and other groups have done, with guerrilla and terror tactics. ISIS still uses them, but their need and ability to operate as a more traditional fighting force exposes them to greater danger than Al Nusra's tactics. It's easier to identify a target in a stolen military vehicle driving down the road than it is to target an old Toyota Corolla.
First of all, this is a myth, and it tells me that you don't actually follow the conflict closely. There is no difference between the way ISIS and Al-Nusra fight. Even those Humvees they stole, they're just using them as VBIEDS. Al-Nusra also have tanks, and neither is fighting like a traditional force. They both rely on VBIEDS, suicide bombers, tunnels, planted explosives and snipers. That's how ISIS fight too.
It's clearly not about the size of the group, those estimates of ISIS "reaching 100,000" include their presence in the whole world, and the CIA put their estimation about ISIS forces in both Iraq and Syria as 20,000-31,000. Al-Nusra estimates are upward from 10,000, and it's not realistic to think it's less than that since they're fighting on pretty much all fronts in Syria.
You're clearly not talking about Al-Nusra here, but about the Khorasan group, which was indeed the primary target for the US strikes, and without the Khorasan group no Al-Nusra fighter would have been targeted. Most of those 136 actually do belong to the Khorasan group and not the main Al-Nusra organisation.
So the numbers don't add up anyway, there is no way Al-Nusra is 30 times smaller than ISIS. Besides, if you're taking every detail into consideration here, then you have to remember that the airstrikes against Al-Nusra were far more concentrated (leading to more casualties per strike) which means if we actually compare the total number of airstrikes, the difference will be a few folds more.
But regardless, if you're now trying to use the excuse "they're embedded so deep in our 'moderate opposition' that we can't target them", then how the hell are you providing those "moderate opposition" with all those dangerous weapons knowing that they're deeply infiltrated by terrorists (at least)?
It's clearly not about the number or the location or the equipments, you know Al-Nusra (and Ahrar Al-Sham and the other terrorist organisations in Syria) are the major forces that are fighting Assad in Syria and have a chance at toppling him, and you know that fighting them will mean the end of the "rebellion", and that's the real reason why you avoid targeting them. You're using terrorism (and funding it actually, umm "indirectly") to achieve your political goals. That's the reality.
In fact, it's exactly what you were trying to accuse Assad of. You're the ones fighting terrorism selectively, and only when it fits your political agendas. Remember this?
Good times.
We are in agreement