Completely normal. On any detector that has ID features, in fact. Same with the visual display. Many things can affect this: soil minerals, proximity to trash item, the item being sensed isn't in the "normal" spectrum of targets, or ground conditions can affect the accuracy, size/depth and position in the soil of the coin.
In truth, these things "fool" the detector.
Wanna have some real fun? turn your SENS UP a coupla notches and listen for one of those erratic, non-repeatable signals. Chances are the TID is bouncing around, not locking onto anything. The detector detects something, but it cant ID it - enough of the signal makes it through the filters to say its this... no wait it's that... and the result is a jumpy TID.
I just remember that the ID can be fooled, but I also know that most detectors are fairly accurate on anything shallower than 4". So if it gives me an indication of, say, a penny - it might be a penny. Or it might be a dime. Or maybe a small silver charm. Either way, I'm on my knees. I consider the ID function as an ID range of possibilities.
Here's a coupla real world examples. I just got my new 250 yesterday, but already I've noticed that it tends to lump dimes and copper cents** together. The belltone sounds and the display reads a notch above the graphic cent symbol - or thereabouts. Usually it's a cent, sometimes it's a dime. Regardless, you'll find me in my usual spot - on the ground digging.
Second, lets talk about where signals are repeatable AND fall into the nickle -> coin range, that "no-mans land" where pull-tabs reside. They often come in strongly, but may not read dead on anything in particular. Usually it IS a tab of one sort or another, since sometimes they read off due to being mangled in mowers, not to mention that there are 20-hundred different kinds.
But, once in a while, you find an interesting tidbit - a foreign coin, or a US coin close to a piece of trash, etc., or some bit of gold, like the gold false teeth I recently unearthed. They read high tab and were in a screwcap infested area.
Try this experiment, too. Turn off the Tone ID feature and place a piece of black electrical tape over the screen. Now all you have is the mono-tone audio to work with. Now you are listening for subtle nuances and believe me, you need to really do some listening! Try this for a half hour each time you go detecting. The first d'tector I ever had - a Tesoro Silver Sabre II - had only this and a DISC knob. I found some great stuff with it.
And no matter what the TID says, dig ALL smooth targets that are deeper than 3", especially if your hunt site history ("HSH", as I like to call it) warrants it.
The TID features we enjoy on modern detectors were only a dream just 20 years ago. The improvements really are astounding - just ask anyone whose been in the hobby that long. But the electro-physics have never changed nor will they and "chaos rules the day" below the surface of the earth.
Detector TID can be fooled. However, the detector really only does one thing - detects the presence of metal. Just consider the TID as a useful range of possibilities and do some digging. You'll soon learn that each site is different and you might be surprised at what you've been missing.
David
** TRIVIA: The U.S. mint doesn't make 'pennies', and never has. They make cents. Have a look at the denomination on the rear of the next 'penny' you encounter - go ahead, take one out of your pocket right now. Yowser! It says ONE CENT, doesn't it?
The term "penny" is a hold over from colonial days when English coinage commonly circulated, and is a Sterling unit measurement - officially it's "pence".
When it was decided that our fledgling nation wanted nothing to do with the English way of doing ANYTHING, we settled on the decimal system which was used by France at the time. The French word for one-hundredth is "centime", thus the word CENT was adopted.
I've been encountering that from time to time, tones that aren't for that coin.