Why do two possible PCR products differ by 300 base pairs?

Answer 1

If the PCR that was run was an RT-PCR then the band with 300 extra bp could be caused by the presence of contaminating gDNA in the reaction. Many primers for RT-PCR are designed to sit in different exons. If the intron in between was about 300bp in length and gDNA was added to the reaction as well as cDNA then two bands would result, the shorter/lighter one from the cDNA and the longer/heavier band from the gDNA.

Answer 2

The difference in size between two possible PCR products could be due to variations in the regions flanked by the primers used in the reaction. If one primer binds closer to the target DNA region than the other primer, the distance between the primers and the resulting PCR product could be shorter or longer. Additionally, differences in DNA sequence, such as insertions, deletions, or mutations within the target region, can also lead to variations in PCR product sizes.