When computing most seats remember that every LAB gain from CON is worth double every LAB loss to the SNP
It’s all a question of simple mathematics
So if LAB lost all 41 of its Scottish seats it would need a further 21 gains from CON to offset them in the race to see which party has most MPs. Quite simply a LAB gain from CON increase the red total by and decreases the blue one. A loss to the SNP simply reduces the LAB overall number.