Most fruit trees that you buy are clones grafted onto a rootstock. A few years ago it was popular to sell trees with two or three different clones grafted onto the one rootstock so that you got different fruits on the same tree. I don't know if they still do this or not.

It can be done by you...grafting a twig from one tree to another

a wedge shaped cut on the twig and a V notch into the twig of the tree ... wrap well and tight with garden tape.

It can also be done with roses

Watched a guy on tv who grafted more than a dozen apple varieties on to one large apple tree and they all produced fruit!.

They are for sale from many garden centres called 'Family Trees' with three varieties grafted on one rootstock. The good point is that the varieties make sure that you get a good polination but the downside is that some of the different varieties can be more dominant than others and need to be pruned more regularly.

If that wasn't advertised on your tree, Sara, it may be a sucker that came from the roots that is fruiting. Another, rarer, possibility is that your tree has had what is called a "bud sport". That's where a cell in a bud mutated, the twig grew, and part of the resulting branch now bears fruit that is slightly different from the original. 'Red Bartlett' pear was first found as a bud sport on a 'Bartlett' tree.