Slopes uses both vertical and horizontal movement to calculate your speed.
Because poor GPS can throw off speeds, Slopes measures a sustaining top speed. In other words, you need to sustain a top speed for a second or two for it to register in the stats. For the vast majority of the time with how people ride, this works out just fine, but if you're cruising at 30mph and zip up to 45 for 2 seconds and then settle back down, it might only register 43 or 44.
This smoothing that Slopes does helps avoid a lot of issues with bad data, but can end up missing quick bursts of speed unfortunately.