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Because your effective duration is calculated on the change in yield of 1% (i.e. bond price when yield is at 6% and 4%, vs. 5%).
In the formula:
105.56 is the price when yield is at 4%
98.46 is the price when yield is at 6%
And your PV0 is when yield is at 5%
So the change in yield is 1% (vs. 5%)
Does this make sense?