In order to make room for stunning new content to shine every day, the Pulse of all new photos drop after 24 hours. After this period of time, even the most popular photos that have hit a 99+ Pulse drop down to anywhere between 40 and 60 over time.
However, it would be pretty unfair to stamp a Pulse of 50.0 on a photo that once hit 99.8, so we’ve decided to show the highest Pulse of an image in any non-timely contexts (such as a user’s profile), because that is the best representation of the overall success of the photo!
Keep in mind that it's also possible for Pulse to drop if votes are removed from it, or voters' accounts are closed after voting for a photo.
For more information on Pulse and Popular, check out What is Pulse and Views.