東西真的滑掉了
這一支就是核心畫面本身，也是離動詞 slip 最近的一條。其他用法都是把這個動作搬到看不見的東西上。
noun · B2
When something slides out of the place where it should stay — snow on a roof, a belt inside a machine, or earth on a hillside. It can also name how far the thing has moved.
滑動；滑落
核心畫面
Picture something that was holding its place and then lets go — snow sitting on a roof until it slides, a belt a wheel until it starts to turn without biting. The thing does not fall away completely. It moves off the spot it was supposed to keep, and you can measure how far. Every use of this word is that same small loss of grip, applied to something else: to a date, to a level, to a price.
可以想成一樣東西本來停得好好的，然後鬆開了。屋頂上的積雪撐著撐著，忽然整片滑下來。皮帶原本咬著輪子，開始空轉。東西不…更多收起
可以想成一樣東西本來停得好好的，然後鬆開了。屋頂上的積雪撐著撐著，忽然整片滑下來。皮帶原本咬著輪子，開始空轉。東西不是整個掉光。它只是離開了原本該待的位置，而且滑了多少量得出來。這個字的每一種用法，都是同一個「抓不住、滑開一段」的畫面，只是換成不同的東西：換成日期、換成水準、換成價格。From the word’s history
The word is slip plus the ending -age, which turns an action into a noun for the act or the amount of it — so slippage is literally 'the slipping', or 'how much slipping there was'.
這個字是 slip（滑）加上字尾 -age 組成的。-age 把一個動作變成名詞，常帶「這件事的發生」或「發生了多少」的味道。所以 slippage 字面上就是「滑掉這件事」，或是「滑掉了多少」。這正好說明它為什麼既能指事情發生，也能指幅度。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1228702528:9320), entry slippage: "From slip + -age."
Three branches
水準或數字往下滑
把「抓不住、滑開一段」搬到一個看不見的水準上。業績、體能、標準原本守得住，慢慢往下掉了一段——滑的幅度通常不大，要拉長時間才看得見。
noun · C1
A fall in a level, a number or a standard — sales, prices, support, fitness or performance getting lower or worse, often slowly and by a small amount.
下滑；退步
從說好的位置滑開：時間、成本、價格
這一支滑掉的不是實物，是原本釘好的那個數字或日期。計畫像被固定在牆上的一根針，鬆了，就往旁邊滑一段。
noun · C1
When a piece of work is not finished at the time that was planned, or does not stay within the cost that was planned. It is used most often about projects, schedules and budgets.
進度或預算的落後