Understanding Bard Song ticks

[peterpal]

I've noticed varying durations when twisting songs. Because of that I tried to experiment with a few songs followed by a /stopsong.
And despite the description in the wiki of 3 ticks (18 sec?) for most songs (Selo's and Restoration for example) I only get the buff 2 ticks (12 sec).
On the other hand songs like Shauri's invis always get the full duration, sometimes even longer.

Is there any rule or reason to know about the real duration of the songs?