This result can be caused by several reasons:
1. The ratio of 9:3:3: 1 is true for an infinitely large number of plant seeds. With smaller samples, the ratio may slightly vary.
2. The shift in the ratio can be caused by chained inheritance, that is, there are groups of chaining genes and they are not recombined because the ratio 9:3:3: 1 is valid only for genes inherited independently.
3. Also, the shift in the ratio can be caused by the influence of some third gene-regulator, which activate one of the pairs of allelic genes in the heterozygous state, or vice versa partially suppress one of the pairs of allelic genes in the homozygous state