Zabbix stores current timestamp before getting and process network discovery rules. If a rule has many IP addresses or multiple services to check it can finish after new next timestamp:

So current process has the following steps:
1. Store current time
2. Perform select query for discovery rules
2.1. Perform discovery rule check
2.2. Update nextcheck field: time from 1 step + rule delay
2.3. Back to 2.1 in case of multiple rules per process