If a blood test reveals that something is borderline high, does that mean something is unhealthy about you? for example: mean platelet volume = 13.0.

Nope: Normal ranges for lab tests are calculated to represent the range within which a given result will be in 95% of healthy people. That means 5% of ALL lab results are outside the normal range even when nothing is wrong at all. So let's take a test panel like a complete blood count where there are 10+ tests bundled together: at least 1 will usually be "abnormal" or "borderline" for anyone.