PCR bias happens when a pcr can amplify 2 or more products and some products amplify better than other products. So if one amplimer is much longer than the other it may melt slower and anneal quicker so the longer amplimer will amplify less well than the short one that melts quickly and does not reanneal easily. Sometimes it is seen in alleles where both amplimers are the same length but one has a CG at one position and the other has AT at that position and the CG is more stable so the AT allele will amplify better as is often seen in dna sequencing or the intensity of peak sizes in using repeat sequences in mapping studies