Why people RETEST and fail the ASVAB after they had scored high the first time?

First of all excuse my grammar since English is not my first language. I took the ASVAB and passed it the first time but still did not scored high enough for Medic Specialist or Linguistics so I retested and failed the test with a score 14. I retested for the 3rd time after being tutored and studying hard for 4 months and failed again with a score of 23. I find it so hard to believe I failed it since I am good at arithmetics, general science and word knowledge and to be honest I found the test super easy. I studied for 4 months with the ASVAB for Dummies, Kaplan study guide and the Aron flash cards.

Everyone I know who passed the ASVAB but retested to be able to chose a specific MOS failed miserably. I met a kid who scored 65 and retested and got a 16 and another one who got a 7. I just cannot comprehend how can someone failed a 3 hour test with such low scores after passing the first time with high scores?

Does MEPS failed people who retested after passing because they consider that cheating? Can someone give me a advice or tell me if you've had been in this situation? Did you finally passed?

Thanks