It had to do with some way to scale cpu's back then. To compare amds to intels, use to be mhz was the stantard in comparing cpus but when amds at 2400mhz performed better or the same as the intel P4's at 3800mhz they labeled them as 3800+ in an effort to inform people that their cpu was yes "slower" in mhz but was actually faster or equal
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More Advanced Micro Devices than Intel