It isn't... a 455 with a good barrel shoots like Yo Dave's 457. What seems to have happened, is that since the beginning of the 455 era, CZ quality control has declined. More frequently than occurred with 452s, 455 barrels had brutally cut crowns and/or crappy chambers from a dull reamer. Sometimes they change the tooling, and some good barrels are made. A few reports are coming in over at RFC, and people are having accuracy issues with the 457. They haven't figured out why yet, but ignition issues seem to be the prime suspect. One guy had his crown inspected and reports that it is brutal, needing to be redone. These barrels with machining flaws would shoot just as bad if they were installed on a 452.
If you buy a match grade barrel, your accuracy potential is the same whether you fit it to a 452 or a 455, it'd just cost more to fit it to the 452.