Hi guys, 

I have seen A few people have problem with throttle, where the quad just doesn't level when it's going up, although they have done ESC calibration many times correctly.

And then some people suggest to make sure in BLHeliSuite settings, the PPM min and max  values should match up (the same) on all 4 ESC's..... 

I cannot find much info about this and would like to learn more, can someone explain the process and the reasons for doing it? or can anyone help a layman understand how and why its done please.

The throttle signal from the receiver and the throttle signal the FC gives to the ESCs is completely independent. 

The FC generally expects the input from transmitter to range from roughly 1000 to 2000, with 1500 being centre stick. 

The output the ESC's expect is set by the calibration.

Every time you calibrate your ESC's, your ESC remembers what the range of your TX throttle stick is

By calibrating your ESCs using the flight controller, you should get the maximum performance.

But sometimes the PPM throttle Min/Max values on 1 ESC might be slightly higher or lower than another, so one motor might generate more or less thrust at higher throttle, that's why your quad isn't leveled at punch outs... By matching all the PPM throttle values can make sure your ESCs are all in sync and produce the same thrust through the whole throttle range.

thank you I still have a lot to learn!

If you calibrate the ESC's from Naze32 (or other FC) and read them back with BlHeli Suite you will see they sometimes don't match up with values on other ESC. Probably due to x-tal clock being different or something like that.
Therefore if you use blheli - you don't need calibrate your ESC's. The cleanflight/betaflighr values for the ESCs are the same that are used in BLheli.