The biochemical processes like DNA Replication are never accurate. Then how the organisms which reproduce via asexual mode of reproduction have offsprings exactly identical to them without any variation?
Mutations do occasionally occur, most of these are small, or in non-used parts of the DNA. Most of the 'real' mutations are 'bad' and probably won't survive in the long term or even be viable from the start. So the original tends to be in a super-large majority.
Some mutations may be stronger, or more fitted to eventually new conditions.
Think of flue-virusses, or MRSA.
DNA replication is extraordinarily accurate. DNA polymerase and other enzymes proofread the new complimentary strands, which are complementary to the DNA template, and when the wrong nucleotide has been placed in the complementary strand, it is removed and replaced with the correct one.
With this system, a cell's DNA is copied with less than one mistake in one billion nucleotides. This is equal to a person copying 100 large (1000 page) dictionaries word for word, and symbol for symbol, with only one error for the whole process!
So mistakes in DNA replication are really very rare, so the offspring produced by asexual reproduction would be expected to be genetically identical to its parent.