[R] log transformation and mean question

[1Rnwb]

I have question about log2 transformation and performing mean on log2 data. I
am doing analysis for ELISA data.  the OD values and the concentration
values for the standards were log2 transformed before performing the lm. the
OD values for samples were log2 transformed and coefficients of lm were
applied to get the log2 concentration values. I then backtransformed these
log2 concentrations and the trouble started. when i take the mean of log2
concentrations the value is different than the backtransformed
concentrations.

> 100+1000/2
[1] 600

> 2^((log2(100)+log2(1000))/2)
[1] 316.2278

What I am doing wrong to get the different values

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