I would guess that this is related to the fact that inbred lines will be mostly homozygous for all variants, which makes variant calling a bit easier – either it's one thing or another thing and anything in low frequency is probably just dodgy sequencing. In outbred species, such as human, there's a lot of heterozygosity which means your variant caller would need to distinguish between all one thing, all another thing or ~50:50, with the added complexity of dodgy sequencing.