Roy,
That's why your resin life is so long. Your system probably removes 98% of the 118 ppm in your tap water, and you're then feeding your resin with product water at only 2 or 3 ppm TDS. If your tap water was, say 400ppm, and you were using a Merlin, you'ld be feeding water at 28 ppm to your resin, and it would only last one tenth as long.
Horses for courses !
Andrew