Why is change since the last trade shown as positive for BUY orders?

The ending balance displays a change relative to the last trade.

For example - you activated a bot with an allocation of $100.

1. Trade #1: Bot bought BTC at $10,000 for 50% of position size ($50/100)
2. BTC had risen to $12,500

         a. Bot balance: $112.5

         b. Current profit = $12.5 (+12.5%)
3. Trade #2: Bot bought more BTC for another $50 at $12,500

         a. Change since last trade: $12.5 (+12.5%)

When bots make partial buys, this can result in change during several buy executions.