I had read an article about three months ago that due to the severe drought that hit Europe last summer the olive trees greatly suffered. The blossoms had bad pollination, the fruit (olives) that did develop were not as large and so on. Based upon 'supply and demand' the supply of olives at harvest time was significantly less but the demand remains high - thus an increase in prices.
Adding other plant oils to olive oil happens all the time with unscrupulous businesses. There was a warning that these types of businesses would take advantage of this shortage by mixing greater amounts of other oils with some olive oil and passing it off as "100% real olive oil." So beware on what you purchase. That cheap olive oil may not be what it actually is. And this can cause problems when making your soap. Different oils within what you think is pure olive oil can really mess-up the saponification process. That may be one reason why different batches of soap behave strangely and not consistently.