Apple and the other companies were originally at loggerheads on the design, but it's probable that a compromise was reached in using the dimensions originally proposed by Apple but with a 'notch', wanted by the others to allow standalone insertion and removal without the need for a SIM tray. We won't know for sure until ETSI releases more information.
From the press release:
At its 55th meeting held on 31 May and 1 June 2012 in Osaka, Japan, ETSI's Smart Card Platform Technical Committee agreed a new form factor for the UICC, popularly known as the SIM card.
The fourth form factor (4FF) card will be 40% smaller than the current smallest SIM card design, at 12.3mm wide by 8.8mm high, and 0.67mm thick. It can be packaged and distributed in a way that is backwards compatible with existing SIM card designs. The new design will offer the same functionality as all current SIM cards.
The new form factor was adopted by industry with the involvement of major mobile network operators, smart card suppliers and mobile device manufacturers. The new design will be published in due course in ETSI's TS 102 221 specification, freely available like all ETSI standards from the ETSI website.