Why do the Big Four PGSes (and GEDmatch) produce sometimes-quite-different lengths for the same HIR? (I.e. Mom and DNA cousin N have both sent spit kits in to each of the four, and share a nice, solid HIR on all four PGSes . sometimes they agree, but other times they don't: on one it's 26 cM, on another it's 28 cM, on another it's 30 cM, and on another it's 32 cM.) They all give the same start and end points (in megabasepairs) for the HIR . but they somehow differently measure the distance between those endpoints. And on what does this variation depend? Is there a pattern, or does it depend on random things we can't see or control for? Does it depend on who did your DNA? I.e. if you send spit kits in to 23andMe and AncestryDNA, then copy both of them to MyHeritage, will the same HIR you share with DNA cousin X (on MyHeritage) be different lengths, and if so, why? (Yes, I know 23andMe's SNP density's lower, but why would affect the HIR's length by so much?) (AFAICT, none of these variations have a material effect on genetic genealogy. I'm just curious.) Best, Eric