That's because the input is not interpreted the same way by Mathematica and W|A.

Mathematica requires precise and unambiguous input. That means that you have the possibility to phrase your problem precisely, but you are also required to phrase it precisely.

W|A is purposefully sloppy. It interprets natural language and makes guesses about what you mean. It's sometimes quite hard to control its interpretation of an input.

It is a common misconception that W|A takes Mathematica syntax. It doesn't—at least it doesn't interpret it the same way as Mathematica would, even if it does interpret certain trivially simple inputs in some way. For the rest, I believe you might as well type English and it won't make a difference.

Solve is meant for symbolic computation with exact numbers. It is not a good idea to use it with inexact input such as yours. Solve (like all other Mathematica functions) also has a specific purpose: solving polynomial equations or equations that can be reduced to such a form.

It appears that you are after an approximate numerical solution. Use the appropriate function: FindRoot.

FindRoot[{0.33*p^0.67/B^0.67 - 2 L == 0, 0.67*B^0.33/p^0.33 - 0.5 L == 0, 100 - 2 B - .5 p == 0}, {{p, 1}, {B, 1}, {L, 1}}] (* {p -> 134., B -> 16.5, L -> 0.671321} *) 

I suspect W|A does exactly that.

Note that FindRoot will return at most one solution.

Alternatively, use appropriate input for the function you are using. 0.33 is not 1/3. I assume you meant 1/3, so let's do this:

Solve[{1/3 p^(2/3)/B^(2/3) - 2 L == 0, 2/3*B^(1/3)/p^(1/3) - (1/2) L == 0, 100 - 2 B - (1/2) p == 0}, {p, B, L}] (* {{p -> 400/3, B -> 50/3, L -> 2/3}} *) 

Note that if you were solving this with pen and paper, you'd find it a lot easier to handle a power of 1/3 (which can be eliminated by cubing) than a power of 33/100.

That's because the input is not interpreted the same way by Mathematica and W|A.

Mathematica requires precise and unambiguous input. That means that you have the possibility to phrase your problem precisely, but you are also required to phrase it precisely.

W|A is purposefully sloppy. It interprets natural language and makes guesses about what you mean. It's sometimes quite hard to control its interpretation of an input.

It is a common misconception that W|A takes Mathematica syntax. It doesn't—at least it doesn't interpret it the same way as Mathematica would, even if it does interpret certain trivially simple inputs in some way. For the rest, I believe you might as well type English and it won't make a difference.

Solve is meant for symbolic computation with exact numbers. It is not a good idea to use it with inexact input such as yours. Solve (like all other Mathematica functions) also has a specific purpose: solving polynomial equations or equations that can be reduced to such a form.

It appears that you are after an approximate numerical solution. Use the appropriate function: FindRoot. It uses direct numerical schemes such as Newton's method.

FindRoot[{0.33*p^0.67/B^0.67 - 2 L == 0, 0.67*B^0.33/p^0.33 - 0.5 L == 0, 100 - 2 B - .5 p == 0}, {{p, 1}, {B, 1}, {L, 1}}] (* {p -> 134., B -> 16.5, L -> 0.671321} *) 

I suspect W|A uses FindRoot for this problem.

Note that FindRoot will return at most one solution, and that by their very nature numerical methods won't guarantee the completeness of a solution.

To use symbolic methods, provide appropriate input for the function you are using. 0.33 is not 1/3. I assume you meant 1/3, so let's do this:

Solve[{1/3 p^(2/3)/B^(2/3) - 2 L == 0, 2/3*B^(1/3)/p^(1/3) - (1/2) L == 0, 100 - 2 B - (1/2) p == 0}, {p, B, L}] (* {{p -> 400/3, B -> 50/3, L -> 2/3}} *) 

Note that if you were solving this with pen and paper, you'd find it a lot easier to handle a power of 1/3 (which can be eliminated by cubing) than a power of 33/100.