Even "real" truffle oil (or butter or some other fat) need not contain real truffles. It's quite easy to infuse fats with truffle aroma. If you put truffles and a fat (with sufficient surface area in contact with the air) in an enclosed place like under a bowl in a refrigerator, you will get a very significant truffle aroma/flavor in the fat.
I've done this with Oregon white truffles (tuber oregonense) that I harvested with the help of my dog. Kept the butter frozen for over a year, and it still had the truffle aroma when I pulled it out of the freezer.
I actually think the infusion method adds a lot more flavor/aroma than adding pieces of truffle, but the pieces are clear evidence that what you bought actually came in contact with real truffles--it's pretty much just marketing.
Truffle oil can be produced using any oil. Common versions use olive oil, or a more neutral flavorless oil such as canola or grapeseed oil.[6]
Some truffle oils are made with truffle residues incurred during collection or preparation for sale. Many truffle oils sold in retail markets are not made from truffles[1] but instead use manufactured aromatic compounds including 2,4-dithiapentane (one of many aroma active compounds that can be found in some truffle varietals) with an oil base. There are no regulations regarding the labeling of 2,4-dithiapentane and it can legally be called truffle aroma, truffle flavor, truffle concentrate or other similar terms, even though it is not extracted from truffles. In the United States, the ingredient may use the modifiers "organic" or "natural" as long as the components meet the federal requirements for those terms. Truffle oils range from clear to cloudy, and yellow to green, depending on the base oil used. Some include a piece of truffle in the bottle.[2] These pieces can be from any of over 200 different truffle species and may be listed as "black truffle" or "white truffle" even if not one of prized culinary varietals such as the black Périgord or white Alba truffle.
This is historic and no longer the case:
Truffle oil is frequently used as a bait for truffle-hunting dogs and pigs. Modern Italians often use a strufion, a ball of rags scented with truffle oil.[7](p87) Truffle oil has been used for this purpose since at least 1756, made by boiling truffles in olive oil and given to hunting dogs.[9]
Well, this does say that "some truffle oils are made with truffle residues". Now maybe this is rare in the US, but at least in France it's the only kind of oil that is allowed to be called truffle oil (well, "huile à la truffe"). It could be called "olive oil with truffle taste" otherwise ("huile d'olive/tournesol/colza au goût truffe").
It's really not difficult to find here, and it is absolutely made with real truffle (the broken pieces that can't be sold as truffles). I don't have any at home because I don't care for it, but one I can find at my usual supermarket for example contains "TUBER AESTIVUM (1.1%)". This is known to be a so-so variety of truffle at best. The next-level product (at almost 40 €/L) specifically says that the truffle in its ingredients is "Périgord truffle (Tuber Melanosporum) 1%". This is probably a good enough truffle oil if you like that.
Looking up the products here in Belgium though, it seems like only "truffle taste" oils are available in the regular supermarkets, but that is also clear from the product names ("Bereiding van Olijfolie van Eerste Persing 99,7% met Truffelsmaak").
https://en.wikipedia.org/wiki/Truffle_oil