How does Oracle determine how much to extend autoextend datafiles?

In 12.1.0.2, I have a LMT tablespace set to autoallocate with an 8K blocksize and 22 datafiles. Each datafile is set to initial 256M, extend 256M, autoextend to a max of 31GB. The contents are only heap tables with a few inline LOBs.

Last night, Oracle decided to autoextend three of the datafiles that aren't maxed, a total of 124 times 256MB, which happens to equal 31GB. That's fine, except it only required less than 2GB, presumably by a table expanding.

So, why did Oracle grab a total of 31GB -- suspiciously the exact same as my weird max size of each datafile -- when it only needed maybe 2GB?