i'm not sure i understand. you're looking for help on responding to "we tithe more than our family really has to give and yet we always seem to have enough money"?
if so, well, you said it yourself:
As humans, we always look for patterns and categories and explanations and I'm having trouble recognising that even if something happens more than once, it can still be a coincidence. In my experience, financial matters usually seem to work themselves out somehow and I know that there are any number of explanations for this. It could be purely coincidental that of the people I know, this seems to happen most to tithing Baptists. It could be that they're stretching the truth, outright lying, or only noticing what they want to notice about their tithing as it relates to their financial stability. It could be that these "shouldn't have worked but did" moments are really rare for them and they want to make it sound like they happen more often because they want to convince people that God did it.
your next sentence is, "Hell, I don't know." but seeing as how it follows the part i quoted above i'd say you're wrong.... you DO know. :)
when people have less money they tend to adjust by spending less. there's nothing miraculous about this. and then there's confirmation bias. are they focusing on and remembering the times it has worked out and forgetting or ignoring the times that they've had to make a late payment on a bill in order to force it to work?
are they even tithing at all rather than just saying that they are because it's expected of them? certainly surveys suggest that while people talk about tithing, almost none of them do. surely it's unreasonable to think that you've somehow stumbled across some odd group of Baptists that tithes at a rate that is 4 times that of the typical evangelical (24%) and 20 times the rate of typical adults (5%).
I don't want to respond to them personally; I want to respond to the irrational part of my brain that's trying to go along with them. Sometimes I get stuck in this endless argument with myself and both sides refuse to give up. It's very tiresome and sometimes it helps to hear some perspective from others who may be thinking more clearly, without the emotional side fighting to gain control. Also, it can be helpful to hear an idea that I've already had, but phrased differently so that it makes me think about it in a new way.
Interesting statistics - thanks for pointing those out!