Four stock portfolios called Small$-$LoPrior, Small$-$HiPrior, Big$-$LoPrior and Big$-$HiPrior, and a value$-$weighted stock index that we will treat as the market portfolio. Here $$Small$$ and $$Big$$ refer to market capitalisation, while $$LoPrior$$ refers to a low prior return and $$HiPrior$$ refers to a high prior return $($measured over the preceding $12$ months with a lag of one month$).$ Thus for example the Small$-$LoPrior portfolio is a portfolio of stocks with small market capitalisation and low prior returns over the past year. This statement is about a problem set I have, it consists of data that includes four stock portfolios $($which are describe above$)$ and one market portfolio. Below is a table including the mean excess returns, betas, alphas, and CAPM predicted excess returns:

Table $1$: CAPM Test Jan $1974$ Dec $2023$

Small$-$LoPrior Small$-$HiPrior Big$-$LoPrior Big$-$HiPrior Market

Mean Excess Return $0\mathrm{.}461\mathrm{.}230\mathrm{.}540\mathrm{.}830\mathrm{.}65$

Beta $1\mathrm{.}321\mathrm{.}161\mathrm{.}150\mathrm{.}991\mathrm{.}00$

Alpha $-0\mathrm{.}400\mathrm{.}48-0\mathrm{.}210\mathrm{.}180\mathrm{.}00$

CAPM Pred. Excess Return $0\mathrm{.}860\mathrm{.}750\mathrm{.}740\mathrm{.}640\mathrm{.}65$

Could you answer; Which portfolio has the highest alpha? What could be a possible explanation of this result? Be clear and concise please.