Generally, the body utilizes glucose as energy but if you do not have enough readily available or stored glucose, the body will utilize fat for energy instead. When the body burns fat, it produces ketones that end up detectable in the urine. Even if you're not following a low-carbohydrate diet, exercising regularly or testing with a Vessel card shortly after exercise may show an elevated reading. Also, ketones can be higher first thing in the morning since you are fasting overnight during sleep or if you do intermittent fasting. Plant-based fats are also more readily utilized for energy due to their short to medium chain fat length - they can be broken down easily, similar to carbohydrates. It is the more animal-based fats that have longer chains and are not easily used for energy, and instead, often get deposited as fat storage. Having a good ketone reading or a reading that fluctuates will indicate that your body is utilizing carbohydrates, as well as fat for energy.
Is this the first time your test has shown a high ketone reading, or have you been consistently testing this way?
If this is your first test, I would suggest trying more tests and watch how your ketone levels may fluctuate. If your test is consistently showing a high ketone reading without a low carb diet, lots of exercise, or extended periods of fasting, it may be important to speak with a doctor to determine if this is indicating a more serious medical concern.