When you hit the cue ball it is immediately slowing down, so when you measure your break on a bar table it's going to be faster than on a regulation table. The cue ball starts slowing down from friction, gravity and wind resistance, the further from your cue tip the cue ball goes the slower it goes.
Every time I have been tested it's been on the Sardo table in Vegas, since I worked the booth next to them for years I did their breaking contest countless times. Once I won $250 for the fastest amateur break at the 1st LA Expo and even got a spiffy certificate along with the cash which posted the winning speed of 25.6mph. John Schmidt won the professional contest but forgot what he clocked.