It's difficult not to suspect that there was a conversation which went something like this...
Starz: "We loved Children of Earth..."
RTD: "Good, I've got a great high concept idea in a similar vein, another five parter..."
Starz: "We were thinking ten..."
RTD: "Did I say five? I meant ten..."
Whereas the correct answer would have been...
RTD: "10, That's fine, I've got ideas for two five parters..." [Later, after the meeting, into his phone] "Boys, they want ten! Anyone got an idea for a second five parter? Said I'd be back with full details before lunch tomorrow..."