They are not mapping your full genome each time. They are looking at specific genes that are known to be linked to certain conditions or attributes. When they release a new test, they are sampling more genes and therefore can find more potential conditions or attributes about you.
They ran an exome sequencing pilot recently, so sequencing is certainly coming.
They do genetic testing, not genetic sequencing. They don't have your whole code.
It isn't entirely fixed, but what's changing are 23andme's tests: both genotyping and sequencing prices keep falling, so periodically they can upgrade the chips they use to ones which either cover more SNPs or improve the error rate (>99.9% accuracy still means a lot of errors on scores of thousands of SNPs) or both.
They aren't giving you the results of a complete sequence ($$$). They are looking for genetic variants. So newer tests will look for additional variants.