High level went low without insulin

Hi all
Today I did a few more tests than normal almost a curve and something was strange. Oscars reading at 9.30hrs after AM shot was 413 but then at PM preshot it was 321. Why would his level reduce even though he hadn't had his insulin.

Thanks for any advice. I do have a spreadsheet if anyone wants to browse. Oscar is being increased on insulin gradually because he got very ill once before. His levels are high at the moment.