While not for certain, you almost certainly mismeasured. I strongly recommend you get a friend or parent or SO to help you measure, and do it face towards the wall and hugging it. This is how they do it in the NBA. You turn your head 90 degrees to the direction you’re measuring, with the other hand just gently touching a corner of the wall perpendicular you’re measuring against.
I mismeasured my wingspan the first time too. At first I got 176cm, but that’s because I did it with a tape measure in the air and I just expanded it as much as possible — this will underestimate by a full 2 inches or more. After doing the wall method I am 183 before stretching my arms and 184 after. I measured it 10 times over a few weeks to be sure, and it’s always been within this range.
Also important to measure is actual arm length. You can spot check this in photos because it’s harder to measure, by looking how far down your thighs your hands go. Hands being about 40% of the way from hip to knee is kind of average, below that can look short armmed. It can put your wingspan in perspective, for example my hands can easily touch my knees if I drop them ever so slightly, my hands go down to 70% of my thigh, hip to knee. Someone with wider shoulders and shorter arms could also be 183.5cm wingspan and look way shorter armmed. Shoulder widths (clavicle length) have a Standard Deviation of about 2.5cm or 1 inch, with an average clavicle length of about 41cm. I know this is verging on obsessive, but proportions are complicated
there’s not much info about this on this forum so I figured I would bring it up not only for your sake but for others. Even if you truly have a -6cm ape index right now, if it’s due to small shoulders you might look fine!