This is not due to compression, it's due to consolidation. Orion (like most other NMS systems) is designed to consolidate data over time to keep the database from growing out of control.
The settings on how frequent data is consolidated can be configured in the webUI Admin --> Polling Setting, under there you will see Detailed Statistics Retention, Hourly Statistics Retention, and Daily Statistics Retention. Keep in mind that storing too much data at too detailed of a level can cause database performance issues.
When detailed data is consolidated into hourly data it grabs one data-point for the min, max, and average for each hour and only keeps that, the same thing when it does daily consolidation.
As far as I am aware there is no good way to recover this data.
The other thing you can do is adjust your Sample Interval in the graph you are viewing to match the data that is being retained for the longer time frame, probably set it to one hour and things will look better. However, this doesn't change the data that is being retained, you need to do the above mentioned process for that.
Actually I tracted down a Senior SA who told me the same thing...
I am going lok at what our setting actualy are sinc eyou told me where to look.
Thanks for the quick response!