What is the different bewteen the minimum attenation in butterworth and Chebyshev filter?
The two filters both roll off at n*20dB/dec, but what this means is asymptotically. You'll start to see the roll-off for f&gt;&gt;fstop. The Butterworth has a shallower roll-off before fstop and the Chebyshev has a steeper roll-off before fstop. That's why to attain the Astop you need a lower order filter with a Chebyshev design.
Look at the two plots below, from your specs, and compare the slope at f&gt;&gt;fstop (f=2..3kHz) -both at n*20dB/dec- But their respective slopes around f= 1..1.2kHz are different from their asymptotic slope.
You have made a good observation that has plagued many who look closely!
Calculations and images from https://tools.analog.com/en/filterwizard/
 
Compare a Butterworth-5 with a Chebyschev-3, they both have Astop = -20dB. But the response is not exactely (or mathematically) the same, and practically speaking which one you choose depends on how much ripple you can tolerate in the passband (0..1KHz) and how much attenuation you want around the cutoff.