BMI calculator says i'm underweight, but in the healthy range? huh?

Based upon your current Body Mass Index (BMI) of 18.8, you are currently classed as underweight.

A healthy BMI for your gender and age is between 18.7 and 26.4 and a reasonable weight loss rate is 0.5-2 pounds a week.

maybe i'm just stupid, but this isn't making sense to me. any thoughts? thanks!

That's an inconsistent statement.  The following might be more helpful

• BMI over 25 = 'overweight'
• BMI between 20 and 25 = 'normal'
• BMI between 18.5 and 20 may be a healthy weight for the individual if they have always been very slight, if they eat normally (typically 2000 cals a day) and if they are in good physical shape.  However, if the individual has lost a lot of weight, if they eat very little or over-exercise  to maintain this weight and if they have signs of health problems e.g periods stopping, anxiety, frequent minor illnesses, eating disorders, then they are too thin
• BMI below 18.5 = 'clinically underweight'