A.According to the chart makers, VFR sectional charts depict airports with hard surface runways as airport patterns inside standard size circles or as distinct patterns reflecting runway layout. They point out that all runway lengths depicted on the charts are to scale. The largest airport pattern size used on a sectional chart (scale of 1:500,000) will allow a runway that is 8,069 feet long to be depicted inside the standard circle. Airports with runways greater than 8,069 feet in length are depicted as distinct runways without circles. Exceptions to this include a few multiple-runway airports with runways of less than 8,069 feet that are shown as a distinct pattern. The reason is that because of the layout of the runways and/or the distance between them, it would not be possible to depict them inside the largest circle and still have them be recognized by the pilot.
This begs the question: Why create the airport circle so that only 8,069-foot runways can be depicted? Airport pattern circles depicting runways with lengths greater than 8,069 feet would be too large, cluttering the chart and masking underlying cultural data such as roads, railroads, transmissions lines, and towers surrounding the airport and considered important to the pilot.
Here's the second part of the answer. Runway lengths are shown on the chart, in the airport data block, to the nearest 100 feet. But the rounding-up point is 70 feet instead of 50 feet. This means that a runway would be at the most 30 feet shorter than the depicted length, rather than 50 feet. The rationale behind this is that it would provide an extra margin (20 feet) of safety for the pilot.
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