Why do we expand tuples in execMain.c?

Hi,

I noticed
if (HeapTupleHeaderGetNatts(tuple.t_data) <
RelationGetDescr(erm->relation)->natts)
{
copyTuple = heap_expand_tuple(&tuple,
RelationGetDescr(erm->relation));
}
else
{
/* successful, copy tuple */
copyTuple = heap_copytuple(&tuple);
}

in EvalPlanQualFetchRowMarks, and I'm somewhat confused why it's there?
If it's required here, why isn't it required in dozens of other places?

Greetings,

Andres Freund