...the olive oil is a little bit old - maybe 2 or 3 years old....
The other thing that hasn't yet been discussed is this older olive oil. Fats naturally break down into fatty acids and glycerin as they age and with exposure to warm temperatures and light and perhaps moisture, bacteria, metal contamination, etc. This means the "free fatty acid" (FFA) content naturally gets higher with time as a fat ages. Some fats age slower (coconut oil being an example), and some age faster (the liquid oils).
Eventually these free fatty acids will oxidize further into smelly chemicals called ketones and aldehydes and the fat will then be rancid. BUt you can have a fat that has high FFAs yet not be rancid, which may be the case with your olive oil.
If you use an oil with a high amount of free fatty acids to make soap, these FFAs react with the lye almost immediately. The end result is your soap batter will trace a lot faster than if you'd used the same oil with a low amount of FFAs.
As far as measuring by volume rather than by weight -- If you use a recipe based on weights but measure the NaOH by volume (fluid ounces), you'll end up with a LOT more NaOH in your soap than you want. Vice versa -- if the recipe is based on volume and you measure by weight, you'll end up with a lot less NaOH than you want.