I got a bunch of new belts for Xmas. Today I went to grind out some bad scratches from a blade. I have been using 36 grit to do my first grinds. For scratch removal I tried some new 60 grit belts not wanting to be quite as aggressive, but it sure seemed like the 60 grit belt cut better than 36 have been. Does that make any sense? Is there a rational reason why 60 would be better than 36, or is it maybe just better belts (niether are high end, just cheapy 10 pack belts as I figured I'd burn through the low grits quickly)?

New belts always cut more aggressive - depending on what your comparing them to - even different grits that have worn down even tho they are a "courser" grit. Experience will let you know very quickly.

A worn belt does not change a 36 grit belt to a finer grit, you just have a dull 36 grit belt.

Often belts with finer grits will remove more metal.  This is because of many more "teeth" on the finer grit belts.  So even a larger "bite" by each tooth will not always make up for the much smaller population of "teeth".  Couple this effect with much more work to remove deep scratches and you can see why coarser grits are not always the best choice.  When I want to remove lots of metal fast, I usually use a 60 or 80 grit ceramic belt.  

Thanks gents. To be clear, I was comparing a brand new unused 60 grit to brand new unused 36 grit. I basically would start a new belt for each new project because the 36 belts seemed to get beat up pretty quick. The higher grits are lasting longer (but I'm also working them less). Bigfootnampa's explanation makes sense. I think I'll likely relegate the 36 grits I have left to pre forge cleaning off caked up rust etc. and go to using the 60s for metal moving.

This is a broad topic however suffice to say not all belts are created equal.  I was fortunate enough to have been gifted a number of different high end belt types by John Emmerling a couple of years ago. This started a quest to find decent belts. I live on the southern side of the dark continent but still I've been able to source really high grade ceramic belts and haven't looked back. 

In the case of naturally occurring minerals - sourced, graded, and sized - there can be a wide range of mechanical properties presenting in the finished product.

The same is likely true, to perhaps a lesser extent, for engineered abrasives, all processes and batches not being equal.

You have two issues, one is the quality of the abrasive particle as mentioned by others. Higher quality will keep it's edge longer.

Another is the size. Just like a cold saw blade needs to be changed according to the material you are cutting to keep a minimum of 3 teeth on the cut and a maximum of 6, if you are grinding say a one inch square surface at the time, there is a ratio between the surface and the grit. 

It seems that your 60 grit is the limit you can use and that a coarser grit means too much pressure on each particle (since there is less of them) and therefore the particle gets dull too quickly. 

I use a linisher alot in my work and have found that Zirconiated belts last and cut the best. these are made for stainless steel and I have found that a 60 grit is a good all round general purpose belt