Here the two slots are wasted on each cycle, which is 50% in terms of Pipeline Slots. But in terms of Clockticks, the stall metrics will be 100% since on each cycle there is some stall. Moreover, on each cycle there may be stalls due to different reasons, which means that metrics measured in Clockticks may overlap.
So, metrics measured in Clockticks are less precise compared to the metrics measured in Pipeline Slots since they may overlap and their sum at some level does not necessarily match the parent metric value. But such metrics are still useful for identifying the dominant performance bottleneck in the code.