Some times when the reading is so far off the charts it breaks the test... Your adding drops of ammonia right to the test tube being an example... Where there the ppm is just too high for the test to have enough reagents to react with...
Other thoughts just in case you didn't think about it... After you add the ammonia to the 10g you should wait an hour with good agitation,or longer without for the ammonia to disperse into the tank before taking a reading.
Perhaps the ammonia off gassed from the storage jug, I.e. lid not put on tighly enough. No idea if this can happen with ammonia, but it does happen with alcohol based liquias like cooking extracts and paint thinners, you'd need another sample of ammonia to compare it to though to check this
I'd shake the reagent bottles before use... Not crazy like the nitrate test, but it can't hurt, also dirty test tubes can skew resulta as well