Actually I-17 was designed as a 2-up sloop boat without a spi.

It originated in Australia, where it was lighter as well.

Then things changed when it was brought to USA and Europe.

In chronological order

I-17 (Aus)
I-17 (EU, US no jib, little more weight, later spi added)
I-17R (USA only, taller carbon mast, more sailarea, recently introduced to Eu but replaced again by F17)
I-17XL (EU only, more sailarea on same EU alu mast)
I-17 (US, EU with jib but smaller than the original I-17's, Again Australian and Asian I-17 fly a different jib)
F17 (US, same as the I-17R really only now you have two spinnaker sizes to the boat)

Wouter