Farmr - "efficiency" only 50%

I’m using Farmr to monitor my farm - just wondering about one of the parameters “efficiency”

Mine fluctuates between 50% & 100%

Does anyone know what this refers to, if its an issue or how to drill down into in the logs?

I’m not seeing any major errors or issues in the logs. I’m part of a pool with partials all valid although I haven’t hit a block in a long time