I'm not sure what is the difference in these Ranks. The Lucy writeup, including the raw data, are identical except for the spell name and spell ID.
Yet when I cast Rank 3 in the PoK, where buffs don't tick down, the buff icon shows duration varying between 18-24. Try it yourself if you have the spell. At 18 seconds, it flashes (like any buff close to its end) but look back a bit later and it will be steady at 21 or 22 seconds... I've even seen 24 seconds.
I'd like to think a Rank 3 would give another tick, since it should have some benefit justifying winning a RAID, especially back when the raid actually required a guild force of 36-54 players! The extra tick would be good enough, since it remains a very valuable spell.