Symphony calculates the message read and attachment download count using slightly different methods, meaning the two counts may not always appear to be in sync.
When a user reads an unread message, a read receipt is generated and the count is increased by one. A message can only be read once - a user reading the same message for a second time will not result in the count increasing.
When a user downloads an image attachment, a preview of the image is generated and the count is increased by two, one for the preview and one for the image itself.
For example, if 3 users are in a chat room and an image attachment is sent which they all download, the total attachment download count will be 6 (3 for the preview and 3 for the image itself).
Therefore, in the scenario above it's quite possible for the attachment download count in a chat room to exceed the message read count - this does not mean the total counts are out of sync with each other.