So the saying goes, "Timing is Everything." This is especially true when sourcing material. In 1856 the US ran out of funding to complete the Washington Monument and halted construction until 1876 (it was later completed in 1884). If they were in a position to blend the stones between the two lots, it would have appeared as one. See the variation within the different phases? Good enough for government work I suppose.
Always order the entire material needed, (including ceramic & porcelain) to complete the job at the same time. If you wind up having to order material a second time, there is no guarantee it will match. Only some factory made tiles have dye lots or run lots to match up to when needing to order more for the same job, but there is no assurance it will be in stock when needed.