An interesting iris

Is this a "normal variant" or a new variety?  I didn't notice it last year, and there appears to be just the one shoot with this flower.  Around the pond we have an abundance of a lilac iris with yellow centres to the sepals (an example in extras), and a small amount of a different purple iris, also seen in the extras.  We should also have yellow flag iris, but although it produces leaves each year, it hasn't flowered for the last 3 years (why?).  And then we have this one.

Normally with these simple irises, there are three sepals, which are the larger "petals", and three petals - the small, stubby ones, often it seems held more upright than the sepals.  In this particular iris, the sepals and petals are almost identical in colouring and size.

If anyone has the answer, I'd love to hear from you.