I don't think the issue is that they don't have the data. I think they're just showing the data at a lower sampling rate.

This chart draws data from three locations:

• https://api.bitcoinaverage.com/history/USD/per_minute_24h_sliding_window.csv
  Updates once per minute, but only has 24 hours of data.

• https://api.bitcoinaverage.com/history/USD/per_hour_monthly_sliding_window.csv
  Updates once per hour, but only has a month of data.

• https://api.bitcoinaverage.com/history/USD/per_day_all_time_history.csv
  Updates once per day, but has all data.

Note how the data that goes back further is less precise.