Of course you do!
"Center_advertisement" is set to 300
So, from server start, at
300, 600, 900, 1200, 1500, 1800, 2100, 2400, ... seconds the center part shows.
Under "chat_advertisements", you didn't make a "separate_group"ing, so all would start <X> seconds after 200.
"This server is running ULX Admin Mod version %ulx_version% by Team Ulysses -- ulyssesmod.net" "200"
200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 2000, 2200, 2400
"Use !votekick <name> to vote to kick someone." "300" -
500, 700, 900, 1100, 1300, 1500, 1700, 1900, 2100, 2300, 2500, 2700
"Please remove all and any thruster or turret sounds." "400"
900, 1100, 1300, 1500, 1700, 1900,
See all the numbers that match on each line?
In a single group under "chat_advertisements", if no other group is created, adv 1 timer hits, then time from adv 1 gets added from adv 2.
As megiddo stated, timer one hits, then the next timer starts.