I don't know about the 7PG17 relay, but in general you need a time delay while other relays may be tripping, as the Trip coil monitor relays would be blind during this time.
This is what I found in the relay catalog:
Quote "These relays incorporate a time delay on de-energisation to keep the relay in an operated condition during temporary reductions in the battery voltage." Unquote.
I guess the 400ms is to provide time for DC supply changeover etc. (without giving rise to unnecessary alarm in the mean time) and is a beneficial feature.
You haven't mentioned what is the difficulty is is posing in your system. It makes learning two way.