Currently, it is by design that when you have Advanced Forecasting Feature enabled, that only transactions under Worst Case and Most Likely Forecast Type are included in the Amount displayed under the Tabs (e.g. Opportunities).
Think of it this way, if a user tag a transaction with item total of $100 and set it Low (Worst Case), Report is:
Low (Worst Case) is 100
Mid (Most Likely) is 100
High (Upside) is 100
If a user tags it to Mid, then in Report, its Low/Mid/High is 0/100/100 and thus tagging High would be 0/0/100.
In transaction, Estimate/Opportunity tab can only present one value instead of all three, so it's picking the Mid value. If a user tag it High, then its Mid value is 0 and this is what the user is seeing.