Generally speaking, dissolved oxygen readings are dependent on atmospheric pressure. This is because the density of water changes with pressure, and this in turn affects the mg/L of dissolved oxygen. Here is a resource describing the “why” behind this. It is also dependent on temperature for similar reasons.
The YSI meter does compensate for changes in atmospheric pressure. Here is a document explaining this. The document also provides advice for troubleshooting when challenges are occurring in BOD testing. One important aspect is to make sure you are calibrating the probe before each day of testing.