Why does the position cost change after transferring stocks into Sahm?

It depends on the position cost filled out by the user when transferring stocks. The position cost is optional, and if Sahm does not receive the original cost basis information for the shares transferred in from your delivering brokerage, then the new cost basis shown on Sahm is based on the stock‘s closing price on the day it was transferred in. The profit and loss (P&L) displayed on Sahm is calculated based on this new cost basis, but it's for visual display purposes only. Your actual P&L on the stock won't be affected by this calculation.

Note: The position cost refers to the buy-in / transfer-in cost