Adobe Photoshop CS6 User Manual page 477

By default, the software uses fractional character widths between characters. This means that the spacing between characters varies, and will
sometimes use only fractions of whole pixels.
In most situations, fractional character widths provide the best spacing for type appearance and readability. However, for type in small sizes (less
than 20 points) displayed online, fractional character widths could cause type to run together or have too much extra space, making it difficult to
read.
Turn off fractional widths when you want to fix type spacing in whole-pixel increments and prevent small type from running together. The fractional
character width setting applies to all characters on a type layer—you cannot set the option for selected characters.
Do any of the following:
To set type spacing for the entire document in whole-pixel increments, choose System Layout from the Character panel menu.
To re-enable fractional character widths, choose Fractional Widths from the Character panel menu.
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