There's all sorts of things that will cause you to see less than 100% charging speeds:
1. State of Charge - The Model 3/Y will only hit 250 kW from ~5-20% SOC. From there max charge rate will taper off appx linearly up to 100%. 40% SOC is around 175 kW max, 60% is around 120 kW max, 80% is around 60 kW max.
2. Temperature - Unless your battery is nice and warm (> 45C or so) you're not going to see peak speeds, either).
3. Supercharger issues - finally, if the Supercharger isn't running at 100% due to hardware issues (issues w/chargers in the cabinet, worn out cable/connector, etc) this will also limit charging speeds.
In your case, since you said you were just topping off and only got 45 kW, I'm going to guess that you plugged in around 80% SOC and your battery was cold.