A first GDP estimate is like a restaurant bill drafted before every receipt has arrived. It is useful, but some line items are estimates. Later, more surveys and administrative records come in, and statisticians update the calculation.
UK second-quarter growth was revised from 0.4% to 0.5%. The change does not mean the economy suddenly produced more in September. It means the Office for National Statistics has a fuller picture of what happened between April and June.
GDP combines household spending, business investment, government activity, exports and imports. Some data arrive quickly; others come later. The trade-off is speed now and refinement later. Waiting for perfect information would leave governments and businesses making decisions with stale data.
A revision is not automatically evidence that the first report was careless. Early estimates serve a purpose. For readers, ask what changed, which parts of GDP changed, and whether the revision alters the broad direction or only the decimal point. GDP is a map, not the territory. Revisions sharpen the map as evidence arrives; they do not change the road people already travelled.