There a number of reasons why the HomesEstimate might be quite different from your current Capital Value. These include:

1. Capital Values are calculated at quite long intervals, usually every three years. The HomesEstimate is re-calculated every month. In your case, the market values for your area might have moved a long way from the last Rating Assesment. Hopefully, the HomesEstimate has moved in line with the market for.
2. Your house may be quite different from the typical house in your area (significantly larger/smaller house, views, newer/older). In such cases, automated valuation models like the Rating Assessment and HomesEstimate can struggle to get an accurate estimate of the market value a property capturing its unique situation. And the Rating Assessment and HomesEstimate may have missed the mark by very different amounts!
3. Your property may have been substantially altered since the last Rating Assessment (e.g. an extra bedroom added, a new garage built, or a piece of land sub-divided off the section). If this alteration has been correctly captured in the council records, the HomesEstimate will reflect the alteration, but the Capital Value won’t necessarily reflect this change until the next round of general assessments.
4. The information in your council record may be incorrect (i.e. wrong house size, number of bedrooms or type of construction). Both the Capital Value and the HomesEstimate may be misled in the calculation to a different degree.