I'm not saying everytime it works this way, I all I'm saying is if you get your slip and say Damn I had a shitty 60' and et that pass but "wow look at the trap that time' you need to consider wheel spin contributed. Sometimes hooking will also give you more MPH as well. If you spin you cannot get a true determination of what your car can do.
Here's one of my slips where traction helped my MPH but this is in the 1/8 where it's expected to cause you have no room to get MPH going.
F=ma F=F1-F2
F2=traction and in a perfect scenario (no wheel spin) F2 would be 0. With perfect traction there can be no better elapsed time or total MPH over the same distances. There are plenty other variables when it come to drag racing but under the exact same conditions, more traction will always give you more MPH. The variances people have from time slip to time slip can be attributed to many other things rather than 60ft time and sometimes a higher mph with a higher 60ft time may be a coincidence with other factors.