A hybrid is two different varieties that have been pollinated together to make a new variety. They do not come true to seed because if you save the seeds, they are not the same thing as when the two separate ones were crossed. Saved seed from a hybrid will usually be like one parent or the other, or sometimes some of the resulting plants will look more like one parent and some will look more like the other one.
Heirlooms are not crossed, they are usually self pollinated, so they will look like the parent plant. The only way an heirloom will look different is if it accidentally crossed with something else.
I hope this helps.
Sandy

I may have misunderstood your question. Are you thinking that heirloom varieties were developed from hybrids (that we got the heirloom because someone crossed a couple varieties to develop it)? If that's what you're wondering about, it's not right. Heirlooms have never been crossed with another variety.