He's still giving up his chance to act in the first combat round (the most important round in most encounters) so that he can act in the second round.
You should try looking at this without seeing the round identifier.
Conan reacts at 1 second, Tass reacts at 2 seconds, Gandalf reacts at 5 seconds, and Ogre reacts at 6 seconds.
Tass intentionally waits until the 7th second so get the drop on the Ogre, who wouldn't be paying as much attention to the little kender.
Since a combat round is 6 seconds, Tass and Conan would be acting simultaneously , instead of Tass acting 1 second behind Conan.
To achieve that, Tass waited for 5 seconds before his first action. He, just like every other character/monster involved, acts every 6 seconds.
Initiative only REALLY matters on the initial (see how similar those words are?) round, when people have the chance to act before anyone else has acted in that encounter.
Subduing, killing, or otherwise removing enemies from combat before combat has a chance to really begin is a major asset.
In an encounter with 5 Orcs and Gandalf, if the Orcs go first, Gandalf gets surrounded while sitting on his wand.
If Gandalf goes first, he Sleeps the Orcs, so he can put on his robe and wizard hat.
In either case, the Orcs will never act more times than Gandalf gets to act, so the order of the following rounds is largely superfluous.