There is nothing wrong. The reason the Crisp Output starts from 
is because the linear MF is chosen and the range of the Output MF is from 0 to 1. If the ruleview(fis) command is entered, one can clearly see the result of Aggregation, where the "mom" Defuzzification Method will be applied to obtain the crisp value.
is because the linear MF is chosen and the range of the Output MF is from 0 to 1. If the ruleview(fis) command is entered, one can clearly see the result of Aggregation, where the "mom" Defuzzification Method will be applied to obtain the crisp value.If the Crisp Output value should starts from 0 to 1, then the range of the Output MF should be adjusted from 
to 1.
to 1. fis = mamfis('Name', "Test_FIS");
fis.DefuzzificationMethod = "mom";
fis
% Fuzzy Input #1
fis = addInput(fis, [0 1], 'Name', 'FuzIn');
fis = addMF(fis, 'FuzIn', 'linsmf', [0 1], 'Name', 'mfA');
% Fuzzy Output
fis = addOutput(fis, [-1 1], 'Name', 'FuzOut');
fis = addMF(fis, 'FuzOut', 'linsmf', [-1 1], 'Name', 'mfB');
% Plot membership functions
figure(1)
subplot(2,1,1)
plotmf(fis, 'input', 1), grid on, title('Input')
subplot(2,1,2)
plotmf(fis, 'output', 1), grid on, title('Output')
% Fuzzy Rules
rules = [...
"FuzIn==mfA => FuzOut=mfB"; ...
];
fis = addRule(fis, rules);
ruleview(fis)
% Generate output of Mamdani FIS
figure(2)
opt = gensurfOptions('NumGridPoints', 41);
[x, y, z] = gensurf(fis, opt);
plot(x, z, 'linewidth', 1.5), grid on
xlabel('FuzIn'), ylabel('FuzOut'), title('Crisp Output')
