The Schwarzchild metric has a spacelike singularity, while the R-N metric has a timelike one. The difference between the two physical systems is charge. Obviously you've a very slightly charged black hole, the SC metric is a good approximation because Q/M is too small to really be worried about. However, the change from spacelike to timelike (and vice versa) singularities is not a continous one, it's a discrete one. The discrete nature of the singularity seems to be a fundamental difference (particularly when you're drawing Penrose diagrams, the staple diagram of my black hole course ATM) so I'm having a bit of trouble getting my head around how you could have a good approximation using the SC metric. It seems to me the nature of the singularity isn't a good approximation for that particular part of the system?