rit's Tranquil Was Tanasen still allows interrupts?

http://wiki.guildwars.com/wiki/Tranquil_Was_Tanasen

While you hold his ashes, you have +10...22...25 armor and cannot be interrupted.

On more than one occasion, while holding the ashes, I was interrupted while casting a spell. Is there something I'm not aware of about the functionality of this skill?

Maybe your skills were disabled?

You may also have been knocked down. It would be helpful if you described some situations where this is occurring to see if we can narrow it down to a particular cause.

I encounter this in PVP. I'll check the damage monitor to see what skills popup when i'm interrupted, and update here.

Here are some skills I was hit with:

Teinai's Wind
Meteor
Distracting Shot

I really thought 'you cannot be interrupted', meant you cannot be interrupted, especially for an elite. I can understand knockdown, but still.

Are there any skills that really means, 'you cannot be interrupted', in the true sense?

There are various skills that prevent being knocked down but there is no defense against skill disabling. Sorry. Try using skills that simply have a short casting time.

The knockdowns I can understand but why are those other skills getting through? Even the disabling effect of the shot should be conditional on the interruption happening first.

It might be worth it to file a bug report.

Unfortunately, no. In the game context, "interrupt" means skills that interrupt, and only skills that interrupt. It does not include knockdowns, skills that miss, skills that fail (such as the Assassin skill Black Spider Strike, which must strike a hexed foe, being used on a non-hexed foe), or disabled skills. Skills like Distracting Shot shouldn't disable a skill if the foe is not interrupted, so if they are, that may indeed be a bug.
The last category is a bit tricky, as you'd intuitively assume that disabling a skill means it's interrupted. While the skill is prevented from completing, it's not technically interrupted, per se. As such, Tranquil was Tanasen won't help with them.
A good way to check whether something's an interrupt or not is to have a friend bring the offending skill in a 1v1 match, while you bring either the mesmer's Mantra of Resolve or the dervish's Pious Concentration with an enchantment, along with a long-cast-time skill to serve as easy interrupt bait. Cast your interrupt-prevention skill (and your enchant, if you went the dervish route). If the dervish skill removes an enchant, or if you lose energy when your skill stops casting, then the skill your enemy used to "stop" it is an interrupt.