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Is an addendum necessary for a 10-point increase in the LSAT?
I've taken it three times, and my highest (third score) is in the HYS range, so I'm concerned mainly with how HYS will see the increase.
The lower scores were due to weather cancellation madness/family situation/general anxiety, but I don't want to sound whiny.
Would it be better to not do an addendum at all?

It's definitely not necessary, but may be a good idea since your reason is different than the typical "I studied better" category of potential addendums.