In the end it's true what some said, it's important to take into account that YPG has received the most intense US air support than any other ground force.
I've been following CJTF-OIR reports, not daily anymore. When you look at today's CJTF-OIR reports whilst the large Mosul offensive has been ongoing you will find that the YPG receives CAS/air support on a smaller unit level, meaning they actually pave the way for them whereas for other forces (Mosul example) they limit their strikes to larger targets that affect the allied forces on a higher level.
For example today's report. The number of strikes refers to the amount of separate engagements they decided to take. Whilst in Mosul they strik many with 3 strikes they focused on larger targets, whereas in Raqqah the intensity of engagements is far higher. Basically means paving the road for them to capture the city, this is not just today this has been the case for years. The amount of air support Kurdish units receive exceeds that of other units by far.
http://www.inherentresolve.mil/Portals/14/Documents/Strike Releases/2017/07July/20170705 Strike release.pdf?ver=2017-07-05-071424-433
In Syria, Coalition military forces conducted 33 strikes consisting of 35 engagements against ISIS targets.
Near Dayr Az Zawr, six strikes engaged an ISIS tactical unit and destroyed 11 oil storage tanks, nine oil trucks, six oil stills, six oil separator tanks and a command and control node.
Near Raqqah, 27 strikes engaged 19 ISIS tactical units; and destroyed 17 fighting positions, three tactical vehicles, a vehicle, two heavy machine guns, a mortar system, a weapons cache, a command and control node, a VBIED facility, and a VBIED.
In Iraq, Coalition military forces conducted four strikes consisting of 72 engagements against ISIS targets.
Near Al Huwayjah, one strike engaged an ISIS tactical unit and destroyed a fighting position.
Near Mosul, three strikes engaged two ISIS tactical units; destroyed 33 fighting positions, two rocket-propelled grenade systems, and a front-end loader; damaged five fighting positions and a command and control node; and suppressed a mortar team.
Without this intense airpower they wouldn't be as effective, they aren't the most effective anti IS fighting force out there as many in the media have been claiming. Before this air support IS was pushing them back steadily. That's not the case in Iraq where a collapse caused a complete loss of all the land. Kurds have been overhyped when it comes to their effectiveness in war, it's hard to reverse that in the eyes of the public.