When I split my pension income, it reduces my tax deducted from T4A slip on line 43700, and increases the same amount on my spouse"s contribution on line 43700. Why?

When you split your pension income with your spouse, not only is the income split, but the tax withheld is also split. The same percentage of tax deducted from that pension income that is added to the spouse's tax return is refunded to that spouse. 

From the following TaxTips.ca article: Thus, if 40% of the pension income is transferred to the spouse, 40% of the withholding taxes will also be transferred.

TaxTips.ca Pension Splitting