Ann: SPONTAN achieves positive clinical study results, page-98

Most gaps get filled through normal volatility as well as the belief by traders that a gap will get filled.
However not all gaps get filled. A lot of traders might be selling today thinking they will buy back on the gap closing.
If there are insto's buying, they don't wait for a single set price and expect to get filled at that one price level. E.g. if they want ten million shares, they are less likely to get filled by waiting for a gap to get filled. They might be buying over days, weeks or months to get their fill. That sort of accumulation might stop a stock correcting all the way to a support level or a gap level.
Also, if enough traders are worried about getting back in at the gap price, they might be willing to buy above that price. If traders expect a recovery from a gap, then not many will want to sell just a few cents above the gap price as they would at say 20 or 30c above. Less selling pressure and increasing buying pressure as the sp gets closer to the gap might prevent the gap from being filled.
Yes the gap might get filled but it's not a foregone conclusion!