Pip award renewal

Hi , I received my pip award form a renewal that took 14 months , I was pleased that I continue to get enhanced living and mobility ( with the Help of your website ) I had an assessment over the phone with capita , I was curious to know if it’s normal to award another 3 year renewal and not a longer timescale , as I provided consultants evidence about neurological conditions , multiple sclerosis and immunotherapy treatment for life , they agreed the condition is not going to improve but only warded 3 years when a dwp assistant on my last call suggested that it would be a continued award due to the type of conditions , any advice would be appreciated