Product Family: Act!
Product: Act! Pro, Act! Premium, Act! Premium (access via web)
This will occur if you are using a field type of Character to input currency values, rather than using a currency field.
This is functioning as designed. To sort currency in chronological order, it will be necessary to create a new field and configure the type as currency. All data will need to be manually entered into the new currency field. For more information on creating fields, refer to the following knowledgebase article:
How to create and manage database fields in Act!