if the other ones you are use to older buicks its not the same, the newer ones do slow down faster. all gms after 2000 or so do it, sort of help the brakes i guess. make sure your air pressures are up and the rotors dont look hot. but a slight difference seems normal, you get use to leaving your foot on the throttle a little if you want to coast. the ones after 05 or so when they went to drive by wire are worse, they change the distance you have to leave it, so you have to leave it more on.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock