It could be that neither Hamas nor Islamic Jihad are unified groups. If, within either group, there is a faction of extremists with access to weapons and explosives and they decide to send out some blind fool to play the martyr by blowing himself up and taking a dozen or so civilians with him, then who is to stop them?
The questions remains of course, why do they claim these attrocities?
------------------
"In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move." - Douglas N. Adams