Documentation at http://www.zabbix.com/documentation/1.8/manual/web_interface/configuration#items says the following about units "s":
"s - translated to "yyymmmdddhhhmmmsss", parameter is treated as number of seconds. Only 3 upper major units are shown, like 1m3w5d or 2h4m46s. If there are no weeks to display, only two levels are displayed - "1m5d" (no hours, minutes or seconds are shown)."
GUI shows "1500000" seconds as "2w 3d 8h", i.e. it shows spaces between units.
Server shows "1500000" seconds as "17d8h40m0s", i.e. no weeks and more than three upper major units.