My dad's top number blood pressure went to 300 yesterday. the hospital doctor said that it dropped to 189 and it raised back up again today. why?

Don't matter. C a do: Blood pressure variability doesn't matter. It's been above 200 mmHg, which is abnormally high. Even 189 mmHg is high. There's a natural variability to blood pressure, usually 5-10 mmHg while resting; but these numbers point to something wrong. Hypertension - the tendency for high resting blood pressure - can be set off by salt, diet, even mild "exercise" (not true resting). See a doctor to confirm