I need to give you an example so you understand how the process works. The kidneys filter the blood that carries glucose. Over 180 mg/dl the glucose will spill over into the urine. A nondiabetic animal who has a blood sugar level of 90 will not have sugar in it's urine. A diabetic will spill lots of its sugar over 180 into the urine gradually as the blood goes thru the kidneys. That is why you can have 300 in the blood and only 100 has spilled into the urine. In the AM urine sugar will tend to be higher as more has accumulated over night. If one eats a lot of sugar it will show up in the blood first before any ends up in the urine. The reason why it is important to know what is in the urine is that if the owner is only occasionally checking the blood and the insulin need is changing, such as the body is starting to make more functional insulin then one needs to lower the amount given via the injection or else the blood sugar will become very low in the blood which is dangerous. The body needs the glucose fuel to run normally. So if you are giving insulin and none is showing up in the urine you would want to call your veterinarian before giving another insulin shot.
I hope this makes sense to you now why the two readings need to be different.