Note: the quoted user was me. And here is the reference. To explain why it wasn't 10% in your case, here's an exception from behind the link: "As long as the device maker doesn't change the default settings". So those 10% are the default setting, but obviously can be altered by the "ROM baker". How often that is done I cannot tell; with that post being from 2011, the rule might even have changed meanwhile (though I didn't hear of that either).
So I just applied a little "Google-Fu": Android 4.1.1 still has the comment
If the free storage on device is less than a tunable threshold value (a secure settings parameter; default 10%) a low memory notification is displayed to alert the user."
(emphasis mine). Note the term "tunable threshold", which makes it obvious this threshold can be modified/adjusted. For Android 4.1.1 you can find it at line 67:
private static final int DEFAULT_THRESHOLD_PERCENTAGE = 10;
This is a class variable; but not being an Android dev myself, I cannot tell how it can be overwritten. It certainly will require system permissions (or just an app with the
WRITE_SECURE_SETTINGS permission?). Quoting the latest code:
* This class implements a service to monitor the amount of disk
* storage space on the device. If the free storage on device is less
* than a tunable threshold value (a secure settings parameter;
* default 10%) a low memory notification is displayed to alert the
* user. If the user clicks on the low memory notification the
* Application Manager application gets launched to let the user free
* storage space.
You can see, the 10% rule is still there. But the
DEFAULT_THRESHOLD_PERCENTAGE variable is gone (and replaced by
mMemLowThreshold, which seems to be set in a different class now – see line 361).
TL;DR: The very same 10% threshold still applies, but the "ROM baker" (who creates the ROM) might adjust that. A developer with some more insight might be able to tell if it can be adjusted by other means, e.g. a special app with the
WRITE_SECURE_SETTINGS permission, or even simply via ADB.