Subject specialism is a factor too
Having had the discussion, in a STEM club, about how a physics degree was relevant to an engineering job, I think there is a view being taught by both parents and teachers that GCSE/A-level choices are absolute pre-requisites for specific careers. This may work against the more generalist (and more demanding subjects) like maths, and also present a barrier to someone without an early CS qualification who might consider moving towards that field. Maybe girls select their career choices differently at the point where they start to choose subjects, or are reluctant to move away from their first subject choice at a later date.
Its worth noting that the gender bias only exists in certain cultures, and isn't particularly an issue of technical complexity in a career.