Two main factors determine urine ketones. The first is blood ketones, if blood ketones aren't elevated, they're not available to appear in the urine in the first place. The second is the kidneys. The kidneys can either allow ketones to pass out through the urine, or they can actively increase reabsorption of ketones. So low urine ketones can accompany high blood ketones--not because ketones are being "used efficiently," but because the ketones are being reabsorbed. This makes urine ketones an inexact measure of systemic ketosis.

You really can't tell by looking at the ketone strips whether you're eating too much fat to lose weight.

Low urine ketones don't tell you that you're drinking enough water. High ketones don't tell you that you're dehydrated. Hydration can affect these things, but you can't look at the strips and see that that's what's going on. It's a fairly blunt instrument that can tell you that you're definitely in ketosis, but can't really tell you for sure that you're not in ketosis.