The word "inconsistent" means not always following the same pattern or behavior, often changing or varying in a way that does not make sense or seem logical. Full definition
, using a cheaper product in different circumstances, were almost certain to come up with figures inconsistent with those of their predecessors, and comparisons become difficult, even into modern times. (wattsupwiththat.com)
Fans were understandably extremely cautious after a number of inconsistent results and poor performances over the last month. (anfieldindex.com)
They're also very inconsistent in their performance, from one brand to another and from one use to another. (glutenfreeonashoestring.com)