The Philadelphia zoo offers a 15% discount with early membership renewals. That discount offer ends 2 months before the end of one's current membership. One naturally assumes that a renewal runs from the date of one's current membership expiration. However, the "renewal" is treated the same as a new membership at checkout and runs from the date of payment. So one sacrifices 2 months of current membership for a 15% discount on the coming year. 2 months is about 15% of 1 year, so in fact the discount is no discount at all. This is a highly deceptive practice worthy of being reported to consumer protection authorities.
I trust that you have first taken it up with the zoo's Membership Department. What you describe does appear disingenuous at least but is it the whole story? How did the Membership Dept explain it when you contacted them? Sometimes these things are simply (although inexcusably) glitches in how the on-line membership was coded.
Yes indeed - this is much more likely to be a poorly-thought-out IT system than a deliberate attempt to deceive. I would definitely start by contacting the zoo directly and asking them to check this.
The zoo's website also explicitly says that "if you are renewing a current membership, your new expiration date will be a year from your current expiration date or 365 days from today, whichever is later." So it is almost certainly a glitch or a simple misunderstanding, or it is being done through a "special deals" website rather than the zoo's website.
I also have a Philly Zoo membership. I renewed mine last year on Nov 25, 2020, to take advantage of a deal. Mine doesn't expire until January 19, 2022. Are you sure that's when yours expires? The e-mail sent with my digital membership card has "purchased Nov 25, 2020 at 5:49pm" under my membership ID #, but that isn't the renewal date.