Is it a Bug?
why the 15point font-size, actually display 12.5point
There is a display font-size: 15 points (12.5)
here is the bug file(CS6) down
Not a bug, just the effect of having Object Scaling in your Preferences set to Adjust Percentage
The immediate fix is to select your text box, and Redefine Scaling to 100%. That will change your type specs to show the true size, and allow you to resize it correctly.
Thank you very much
is it can be found in Paragraph style or character style?
Neither. The setting is a preference item, and the reset will be in the Control Panel flyout menu.
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