You have a couple of answers already that give more guidance to the use of the numbers you find on various sites. It seems that buyers heavily weight price/foot as a true and strong factor in assessing the value of a home. In reality, it's only one of the, and one of the smaller values, that I use in that assessment. The true value, of course, is what a willing seller will sell for and what a willing buyer will buy for. Our effort here is to try and determine that value as accurately as possible so we have that seller and buyer coming together, sooner rather than later.
Most of our experience in real estate is built on many factors. The decisions that are made are based on a collection of information, not just one item. Whether leased land is right for you, or a single family home or a condo and community living, along with your budget, future earning power, your age, your famiily status - all are factors in making the home purchase choice.
So, keep your opportunities open by looking at all the components that makeup the whole.
They pull their info for the zestimates from the public records. As an owner you can 'claim' your home and add edit the information to make it more accurate as well to reflect any updates you have made. But if someone has not done this then sites like this are basing on the previous sales and assessing per square footage.
If the home is ACTIVE for sale on the market by a licensed agent than the sales price is set by the seller, usual under counsel of the agent. As previously mentioned, it could be a strategy to draw multiple bids if it seems low. And if set high, it is likely you will see a price reduction or a sales price lower than the list price.
Those sites should be used as just one of many forms of information available to the consumer.