Dave; Per the Bylaws state the following:

"The amount of annual dues is subject to change by the Board of Directors as increased costs and additional services may from time to time require. Change in the dues shall be effective upon approval by the Board of Directors and implemented on a date specified by the Board."

Additionally, dues expire at all different times of the year depending upon what month a member joined ... so July 1st was simply an arbitrary date that the BOD chose would be my guess. I would also guess that most of the dues expire around July 1st due to the annual conventions, where many new members sign up at the club table on the bourse floor.