It's just that some emission reductions are also open technological questions, and sometimes they're more or less open than the questions around sequestration, so hard-and-fast rules are hard to come by. Is it easier to sequester the emissions from space rockets than it is to build a Lofstrom loop? Is it easier to synthesize jet fuel from the atmosphere than to electrify jets? Where should R&D resources be optimally allocated, especially if we're taking a risk that certain approaches won't work out?