Tony Hirst has identified some bugs in the way Google spreadsheets ‘scrapes’ tables from other sources. In particular, when the original data is of mixed types (e.g. numbers and text). The solution is summed up as follows:

“When using the =QUERY() formula, make sure that you’re importing data of the same datatype in each cell; and when using the =ImportData()formula, cast the type of the columns yourself… (I’m assuming this persists, and doesn’t get reset each time the spreadsheet resynchs the imported data from the original URL?)”