What's Hard to Call?

It looks like heterozygous base pairs are harder for 23andMe (or its Illumina chip) to call than homozygous base pairs.

I tested with 23andMe twice.  Of the autosomal SNPs that were no-calls on one of my tests but were genotyped on the other test, about half were heterozygous and about half were homozygous.

This is in contrast to the fact that about 68% of the autosomal SNPs overall were homozygous.

So my heterozygous SNPs were disproportionately represented among the no-calls.