The nomenclature of England is a foggy thing, cunningly designed to confuse foreigners, who will wonder, in their simple foreign way, why they're consuming a Devonshire cream tea in Devon, why an earl's wife is a countess, why Nottingham had a sheriff, and why the Welsh Marches rather than march.
Thank God that I'm an Englishman, and was therefore born confused, rather than having to become so, like a mere Frenchman.
The explanation though is reasonably simple.
Once upon an Old English time there were shires: Hampshire, Wiltshire, Nottinghamshire etc. The Anglo-Saxons lived in these and kept the Hobbit population under control.
Each shire was ruled for the king by a shire-official, or shire-reeve, or scir-gerefa, or sheriff. That's why there was a Sheriff of Nottingham. He would, in fact, have been sheriff of Nottinghamshire.
The Sheriff was therefore an Important Chap, and the Old English word for an Important Chap was an Eorl, or Earl.
Then, in 1066, the Normans invaded and Frenchified everything. The Normans like their faluting to be high and their pants to be fancy, so they decided to call Shires by the Latinate word county.
The head of a county should of course have been called a Count, but he was already being called an Earl and it was hard to change.
Therefore a Norman might consider himself to be the Count of the County around Oxford, but the peasants all called him the Earl of Oxfordshire. The peasants won in the end, because they were speaking English, which is a much better language than all the others.
So that's why England has counties but not counts; the counts became earls, because that's what the peasants called them. But the wives of the counts never went near the peasants at all, and that's why they're still called countesses.
The wonderful English language had, of course, to be protected from its natural enemies like the Scots who wanted to pronounce every vowel as "ae", and the Welsh, who had a language based entirely on cheating at Scrabble. To do this there were particularly militant border counties called Marches.
An earl in one of these shires could have called himself the count of the county, but he preferred to sound all military and tough and point out that he ran a march. So he called himself a Marquis and he called his wife a marchioness.
A March, by the way, has nothing to do with marching, but it is vaguely related to a bookmark as they both mark your place. Moreover, counts don't count.
N.B. I have slightly simplified history so that it conforms more perfectly to etymology. Truth is far preferable to fact. The facts are rarely elegant, and should therefore be ignored.