Picture the baseball park itself: a big fenced space, the field in the middle, seats all around it. Everything inside the fence counts as in the park, and a ball that clears the fence is gone. Notice how loose that inside is — a ball can land near the middle, or way out by the back wall, and it is still in. That looseness is what American English borrowed when it started talking about numbers: a guess that lands anywhere inside the fence is close enough to keep working with.
可以想成一座棒球場：一圈圍牆，中間是球場，四周是看台。只要還在圍牆裡面就算「在場內」，飛出去就沒了。要注意的是，這個「在裡面」很寬鬆——球落在中間，或落在最後面的牆邊，都一樣算在裡面。美式英文後來把這份寬鬆借去講數字：一個估計只要落在那圈圍牆裡，就算夠接近，可以繼續談下去。