Rollup fields do not show accurate results when Base currency of the organization and Default currency set by an user differs.

PRE REQUISITES:
=======================
Navigate to Settings > Business Management > Currencies
Click on New
Added currency Hong Kong Dollar with exchange rate 5.47 (base currency of org is Australian Dollars).

Navigate to Personal Option > General Tab
Set Hong Kong Dollar as default currency.

Navigate to Settings > Customizations > Customize the system
Opened Opportunity form
Created a new roll up field (New TA) in Order form
Source: Order
User Hierarchy: No
Related Entity: Order Products (Order)
Aggregation: Sum of Amount.
Added the field to the form.
Save and Publish customizations

REPRO STEPS:
============================
Navigate to Sales > Orders
Click New
Entered required fields
Saved the record
In Order Product sub grid, Click +
Select Wright in products
Enter details like Name,
Price Per Unit = HK$ 100
Quantity = 1
Again, in Order Product sub grid, Click +
Select Wright in products
Enter details like Name,
Price Per Unit = HK$ 100
Quantity = 1
Saved the record
Refreshed New TA field
The result is HK$ 199.98